A substance inhibiting DNA synthesis in mouse leukemic cells was isolated from the regenerating calf spleen. When added to a suspension of leukaemic cells, this substance is adsorbed on their surface. The following changes in cell features being noticed: 1) a minute decrease in electrophoretic cell motility, 2) a decrease in esterase activity of the cells, 3) an increase in microviscosity of membrane lipids, 4) an increase in the intracellular pH values. With a longer contact with this substance, changes in nuclear chromatin structure were noticed, with special reference to weakened bonds between DNA and proteins. The data obtained are of significance for revealing molecular mechanisms of hematopoietic mediator action on target cells.