Recent studies in this laboratory have been directed toward a more complete understanding of the nature of a lymphoid leukemia virus isolated from the mouse neoplasm, Sarcoma 37 (42, 48).

This virus produces a generalized lymphocytic neoplasm in 100 percent of the recipients within a relatively short period when inoculated into mice of various ages and strains and into rats. The disease produced is similar to that which occurs spontaneously in certain inbred strains of mice.

Biological Properties. Source Of Leukemia Virus

Sarcoma 37 was transplanted from strain A/LN mice 3 to BALB/c mice. It was carried in this inbred line for three tumor generations before the tests were carried out which revealed the presence of a virus.

Sarcoma 37 is maintained as a routine passage series in BALB/c mice and still yields the virus after 50 transplant generations. Such tumor material can be used as a source of the leukemia agent. Additional sources of the virus employed in the studies to be described were: (1) leukemic lymph nodes, thymus glands, spleens, and livers from mice which had received cell-free concentrates of Sarcoma 37 and (2) leukemic hematopoietic organs from mice which had received whole-cell intraperitoneal implants of leukemic tissues, originally derived from cell-free concentrates of Sarcoma 37. The virus-induced leukemia is freely transplantable in adult mice within a given strain. One hundred percent of the recipients develop generalized lymphocytic leukemia in 1 to 2 weeks after intraperitoneal inoculation of the neoplastic cells.

3 Mice of strain A/LN bearing Sarcoma 37 were received from Dr. M. K. Barrett of the National Cancer Institute. The sarcoma had been carried in this strain of mice for 195 transplant generations. No leukemias had been observed by Barrett in the tumor-bearing animate.

Recovery Of Virus From Source Tissues

The techniques used for the recovery and concentration of the leukemia virus from the source tissues have been illustrated and described in detail in a report published earlier (43). In summary, they involve the processing of source tumor tissue by two different procedures, after preliminary extraction and clarification by the following method: A 10 percent homo-genate of tumor tissue in 0.15 M potassium citrate was treated with hyal-uronidase. The homogenate was then cleared of nuclei and cell fragments by low-speed centrifugation. The supernatant (fraction S1) recovered from this run was further fractionated by either of the following procedures:

Procedure A

The particulate fraction containing the virus was sedi-mented from the recovered supernatant (S1), which had been clarified 2 times, by centrifugation at 30,000 X g for 1 hour. The pellets were re-suspended in 0.05 m pH. 6.8 sodium citrate buffer and clarified. The supernatant was recovered and the pellets again washed and clarified. The supernatant resulting from the last centrifugation was pooled with the previous one. The final product was a concentrated particulate fraction containing the biologically active virus.

Procedure B

Fraction S1 was cleared of large aggregates of cellular material by passage through a Selas XFF filter. The filtrate was diluted and refiltered through an 02 Selas bacteriological candle, shown to be impervious to Escherichia coli. To concentrate the virus, the 02 filtrate was then centrifuged at 30,000 X g for 1 hour. The pelletized material was resuspended in 0.05 m pH. 6.8 sodium citrate buffer, to give the final preparation.