A hamster lymphoid cell line, HCT-2, transformed by human T-cell leukemia virus type I (HTLV-I) was serially transplanted for 9 passages in newborn hamsters. A total of 34 newborn hamsters inoculated intraperitoneally (i.p.) with 0.2-2 X 10(7) HCT-2 cells developed fatal lymphomas with dissemination to various organs within 5-10 days. The growth of i.p. inoculated HCT-2 cells was found to be dependent on the age of recipients: all 21 suckling hamsters inoculated when aged 5-10 days succumbed to disseminated lymphomas within 6-7 days, while 4 of 12 older hamsters inoculated at the age of 15-25 days developed less extensive disease with signs of tumor regression. To investigate the effect of immunosuppression on host resistance, 3 adult hamsters treated with anti-thymocyte serum were inoculated i.v. with 2-4 X 10(7) HCT-2 cells; all 3 developed fatal leukemias in 5-7 days. Irrespective of whether HCT-2 cells were inoculated into newborn, suckling, or adult hamsters, histopathological findings were similar, with frequent involvement of liver, spleen, lungs, kidneys, lymph nodes, blood, and bone marrow. Cells harvested from tumors and peripheral blood of some tumor-bearing hamsters could be readily recultured as cell lines. Chromosome analysis and Southern blot hybridization showed that tumors were caused by growth of HCT-2 cells.