In vivoeffects of anti‐asialo GM1(GA1) serum on hemopoiesis were studied by morphological examination of the blood, bone marrow, and spleen and by measuring the number ofgranulocyte‐macrophage colony‐forming units(CFU‐GM) per femur after injecting of the anti‐serum into mice intraperitoneally. The number of circulating lymphocytes markedly decreased 2 days after injection and then recovered, but circulating granulocytes gradually declined and reached a nadir at 6 days, then rapidly recovered. Granulopoietic activity in the bone marrow was reduced on day 2, but recovered profusely from day 4, followed by marrow neutrophilia on day 10. The spleen showed marked extramedullar granulopoiesis on days 6 and 10. CFU‐GM per femur increased by 24 hours, reaching a maximum value 250% of normal on day 3, then returned to the normal level. CFU‐GM growth was not affected by the treatment of anti‐GA1serum plus complementin vitro.These results suggest that the proliferating compartment of the myeloid series in the bone marrow was eliminated by anti‐GA1serum injection that led to a decrease in granulocytes and an i