A variety of substances can mobilize haemopoietic stem cells (CFUs) into the peripheral blood. In this study the involvement of the complement system in the mobilization process was investigated. Pretreatment of mice with the complement‐activating factor of cobra venom (CoF), which lowered the serum C3 levels to 10–25% of the normal value, could completely prevent CFUs mobilization induced by high doses of CoF, endotoxin (ET) from Salmonella typhosa, inulin, zymosan and the proteolytic enzymes proteinase and trypsin. On the other hand, mobilization induced by the polyanions dextran sulphate and the copolymer of polymethacrylic acid and styrene could not be prevented, or at least affected only slightly. There appears to be a relationship between the extent of decomplementation by CoF and the extent of CFUs mobilization induced by ET. The results indicate that certain agents mobilize CFUs via the complement system, whereas other agents induce CFUs mobilization independent of the availability of complement components.

Liver from 14 day-old fetuses, bone marrow, spleen, and blood from normal adult mice, and bone marrow, spleen, blood, and liver from adult endotoxin (ET)-treated mice were used for isogeneic hemopoietic restoration in lethally whole-body irradiated mice. The number of CFU-S required to prevent 50% mortality of irradiated mice was about 3 for fetal liver, 7-10 for bone marrow, 20 for normal blood and for blood, liver, and spleen of ET-treated mice, and 80 for spleen of normal mice. CFU-S growth curves in femoral bone marrow and spleen showed some variations but the differences in survival of irradiated and protected mice could not easily be explained by differences in CFU-S growth curves. It can be concluded that the CFU-S from peripheral blood, although somewhat less effective than CFU-S from bone marrow, can be a valuable source of CFU-S for bone marrow transplantation.