Culture of normal and leukemic hemopoietic cells was performed in the "agar drop-liquid medium" system. It is shown that cloning of the hemopoietic cells with mitogen does not result in amplication of proliferation of clonal cells, and does not enchance formation of optimal conditions for cell growth in vitro. Addition of blood serum of healthy people, and especially the serum of patients with different hematologic diseases to the liquid medium, leads to an increase in CFU-GM or colonies. It is suggested that mitogens may exert an indirect action on the clonogenic cells in which a colony stimulating serum factor probably takes part.