At this point it would be well to give an idea of the biologic response of mice to X irradiation. Dr. Kaplan has talked about a special and unique case in which thymic lymphosarcomas result after certain optimal treatments with radiation: The irradiation has to be fractionated and whole-body, and there appears to be an extremely high threshold. I do not know what it is for C57BL mice, but for the CBA strain, as reported by Dr. Mole, the total dose is about 500 r. On the other hand, as discussed by Dr. Mole, the generalized lymphocytic neoplasms arise at much lower total doses and appear throughout the life of the animal, whereas thymic lymphomas appear chiefly within the 1st year. (See Mole, R. H.: Brit. M. Bull. 14: 174, 1958.)

There is also a difference in response, as shown by Furth and Upton, in the induction of myeloid leukemias in RF mice, in which 150 r, which does not induce thymic lymphomas, is effective in inducing myeloid leukemia. Thymectomy is not effective, and partial shielding, though slightly effective, does not completely eliminate myeloid leukemia. Also, if C57BL mice are thymectomized, there is a different story: Practically 50 percent are reticulum-cell sarcomas, which develop after 1 year of age, and rather late in life after irradiation.

I would ask Dr. Kaplan and Dr. Gross also, since Dr. Gross has reported on a similar type of work in the C3H/Bi substrain of mice, whether they really believe that there are adequate controls at the present time. For example, Dr. Gross reported an 11 percent incidence of leukemia in C3H/Bi mice after the introduction of extracts from X-ray-induced lymphomas. He found that serial passage was very effective in decreasing the latent period and increasing the incidence. Similarly, Dr. Kaplan showed a 20 percent decrease in the first passage material and indicated that as high as 70 percent was attained after serial passage. The control, it seems to me, should have been done in an analogous manner, i.e., serial passage of material from a similar untreated source during the same interval.

Another question I think should be raised-and I am doing this for discussion-is whether or not there is any question of contamination, in these laboratories, by the agent which is obtained from AK mice. Dr. Kaplan has said that he has AK controls, and it would be interesting to have some idea of how many AK leukemias were tested in C57BL recipients. Dr. Gross has shown that not all of them reveal infective material.

Dr. Kaplan suggested that he is observing an activation of a virus, and I wonder if there are enough data to rule out the possibility of changes in the sensitivity of the host cells. For example, if one irradiates adult mice prior to the introduction of polyoma virus or introduces the virus and then irradiates, they become sensitive to the virus and tumors are induced. I do not know of any evidence which indicates that there is any activation of the polyoma virus. But this is a difficult problem, and some of these answers will be hard to obtain. As Dr. Kaplan suggested, there is no readily available and rapid assay method for leukemia.

Dr. H. S. Kaplan: I am grateful to Dr. Law for his discussion because it affords an opportunity to make a few additional remarks. As far as I am concerned, the evidence indicates that there is a virus that can be extracted from AK leukemia tissue, which will produce an identical neoplasm in the refractory C3H strain.

We have used some of Dr. Gross's passage A agent and have obtained tumor yields in C3H mice in excess of 60 percent in about the same interval that he has indicated in his published reports. Dr. Law said that I confused the acceleration phenomenon of Rudali, Duplan, and Latarjet with Dr. Gross's agent. That is not the case. We have taken the passage A agent, obtained from Dr. Gross, and put it into newborn AK mice, and obtained an acceleration phenomenon with over a 60 percent incidence by about 4 months of age, at which time none of the control AK mice have contracted leukemia.

Dr. Law's comments regarding the radiation threshold and Dr. Mole's work on thymic tumors versus generalized tumors seemed to me to be somewhat beside the point. It is perfectly true that I did not talk about myeloid leukemias, but I also did not talk about mammary carcinoma, salivary-gland tumors, carcinoma of the skin, or other kinds of cancer. This is one particular system. It is the only one that has been characterized with regard to the phenomenon that is under discussion, and I did not mean, even by exclusion, to indicate that the other areas are not worthy of study. But in the time available the subject has to be limited. I think Dr. Mole's data are essentially irrelevant to the present discussion.

As for reticulum-cell sarcoma, we have recently completed an experiment in which we used nonirradiated C57BL mice, thymectomized nonirradiated C57BL mice, and thymectomized irradiated C57BL mice, and the final incidence of this sarcoma was essentially identical in all three groups. There is no indication that a relatively large dose of radiation had any influence whatever on the development of this disease, and I do not believe that there is any basis at the present time for assuming that this is a radiation-induced reticular neoplasm. I think it is a spontaneous late tumor of these mice, and radiation has nothing to do with it.

As far as adequate controls are concerned, I mentioned some that we have at the present time. We have not done serial passage of control material, but we consistently get an incidence of 15 to 20 percent with either fresh material or with cell-free filtrates from an originally radiation-induced tumor which has been carried for a good many years in our laboratory, and we consistently get an incidence of zero with any of the control materials we have used, including cell-free filtrates from normal thymus. I suppose serial passage would be a good idea, but it has not been done.

Through the kindness of Dr. Huebner, we have recently tested some of our animals for polyoma virus and we do not seem to have the polyoma virus in our colony. Whether we have contaminated our C57 with the AK virus, I do not know.

About "activation," I have always used this word in quotes deliberately to emphasize that I do not know what activation means. The virus is presumably in the tissues all the time, yet radiation is necessary to bring out the tumor. I do not know whether I am talking about a change in host sensitivity or a change in the virus, but I would be inclined to think it a change in host sensitivity or host response rather than a radiation-induced change in the virus.