IN VITRO ASSAY OF THE STIMULATORY EFFECT ON BONE MARROW CELL PROLIFERATION OF SERA TAKEN AFTER CHLORAMBUCIL TREATMENT

After administration of chlorambucil, sera taken after 5, 6, 8 and 9 days were found to increase the incorporation of 3H-thymidine into bone marrow cells in tissue cultures. In addition, serum taken after 9 days caused stimulation of 3H-adenine uptake. These changes are like those produced by an antichalone, a factor known to increase cell proliferation. The results paralleled the changes in the number of granulocytes occurring in the bone marrow and in the blood. The stimulation was of the order of about 20 per cent. It seems obvious that chlorambucil does not directly cause the appearance in the serum of a factor promoting bone marrow cell proliferation. It is likely that the phenomenon is due to reduction in the number of the cells.