Factors in human serum affecting the proliferation of normal and leukemic cells.

autoradiography). With maturation of the marrow elements, the tritiated thymidine indices fell below pretreatment values. Serial serum samples obtained from these patients affected the incorporation of tritiated thymidine into DNA in primary cultures of normal bone marrows. In relation to pretreatment sera, samples obtained during the proliferative phase caused a twofold increase in tritiated thymidine incorporation by target cells, and sera obtained during the marrow maturation phase caused a twofold decrease. Identical results were obtained when these sera were incubated with marrows from patients with acute lymphoblastic and acute myelocytic leukemia (> 95% tumor) and when they were assayed with the long-term cultured cell lines. Thus, factor(s) are present in these sera that can affect tritiated thymidine uptake in both normal and leukemic hematopoietic cells.