An in vitro-vivo technique for establishment of cell lines on murine leukemia has been developed. Using this method, suppressive T lymphoblastic leukemia L7811-85, L7212-85, non-T, non-B lymphocytic leukemia L1210-86, B lymphocytic leukemia P 388-86 and Friend erythroleukemia FLCL cell lines have been established. Incidence of leukemia with these cell lines was 100%. Along with the increase of generations of cell lines, cell growth accelerated, generation time shortened and cloning efficiencies rose. A following up electron microscopic observation on L7811-85 and L7212-85 showed that the virus particles were "A" particles in original cells. When they became cell lines in vitro, virus particles increased and transformed into typical "C" particles with budding. An inhibitory activity relevant to leukemic cells on proliferation of leukemic cells has been observed in the supernatant of L7811-85 medium and was regarded as an "autocrine".