In 1957 Friend (24) isolated a leukemia virus from the spleen of a Swiss mouse that had been inoculated, shortly after birth, with a cell-free preparation of Ehrlich carcinoma. The Friend virus is apparently strain specific. Adult mice of the Swiss and DBA/2 strains are highly susceptible, whereas mice of the PRI, C57BL/6, A, C3H, and (C58 X BALB/c) F1 strains are nonreactive.

The "leukemia-like" disease produced by the virus has been described by Friend (26) and further interpreted by Furth and Metcalf (26) as a reticulum-cell disease. It is associated with nonmalignant erythroblastosis, without involvement of the lymphoid or myeloid systems. The disease is characterized macroscopically by marked hepatosplenomegaly. The latent period in susceptible adult mice is relatively short, the spleen becoming infiltrated and palpable within 2 weeks after the intravenous inoculation of virus.

The leukemic nature of this virus disease has been questioned on the basis of the inability of the neoplastic cells to produce solid tumors. However, recently Friend and Haddad (27) and Buffett (28) have reported the production of localized tumors from virus-induced leukemic spleen and liver implants. Buffett and Furth (29), using similar tissues, described the development of a transplantable reticulum-cell sarcoma in Swiss mice.

Physical data on the mouse leukemia virus of Friend show that it is stable for extended periods when stored at -70° C. and when dried in the frozen state. It is inactivated in 30 minutes at 56° C. and when treated with formalin (1:200) or ether.

Electron microscope studies by de Harven and Friend (30) on pel-letized leukemic tissue filtrates and on leukemic spleen, liver, and bone marrow sections have revealed the structure of the virus particles. The observed particles were of types A or C, according to the classification of Bernhard and Guerin (31), and were found to have an average diameter of 78 mµ Similar results were obtained by Dalton et al. (82) in their electron microscope studies of a solid tumor. This latter neoplasm originated from a transplant of virus-induced leukemic spleen cells that had been maintained in tissue culture for varying periods.

The antigenic nature of the virus has been described by Friend (S3). Homologous and heterologous antiserums produced in mice and rabbits, respectively, were found specifically to neutralize infectivity of the leukemia agent. Additional studies showed that serums from mice with leukemias induced by either the Gross or the Schwartz agent failed to neutralize the Friend virus, which indicated no direct immunological relationship between these serums and the leukemia agent of Friend.