Leukocytosis promoting factor (LPF) of Bordetella pertussis and pertussis vaccine (PV) were introduced into C3H/He mice respectively and the kinetics of colony forming cells (CFC) were assayed by modification of Till and McCulloch's method. Uptake of59Fe and3H‐thymidine into hematopoietic tissues was also examined with similarly treated mice.In vitroincubation of bone marrow cells with LPF did not cause any particular changes on the development of spleen colonies. Increase of CFC in peripheral blood was significant in both LPF and PV treated animals.In the PV treated mice, erythropoiesis in the spleen and granulopoiesis in the bone marrow were remarkably accelerated on the 5th or 7th day of the injection. The number of CFC in the spleen showed a significant increase, which may result from stimulative effect of bacterial endotoxin. On the other hand, CFC of the spleen and bone marrow were unchanged or slightly decreased in number by administration of LPF, although a temporary stimulation of hematopoiesis was confirmed on the 2nd or 5th day of the injection. The functional derangement of endothelial cells induced by LPF is considered to be the dominant cause of increased CFC in the peripheral blood. The involvement of plural regulatory systems of hematopoiesis has been postulated, based upon the nresent exnerimental result