AbstractAntisera to mouse brain reacts with hematopoietic stem cells in the mouse bone marrow. We have examined the effect of anti‐mouse brain serum (AMBS) on the development of in vitro colonies from mouse bone marrow cells. The addition of 5% AMBS to the cultures markedly decreased the numbers of colonies formed to an average of 10% of the number obtained with normal rabbit serum. AMBS suppressed formation induced by colony stimulating factors (CSF) derived from three different sources; serum from endotoxin treated mice, mouse L‐cell conditioned media, and human peripheral blood mononuclear cell conditioned media. The suppressive activity was quantitatively recovered in the IgG fraction of AMBS. Divalent F (ab′)AHBS, rabbit anti‐human brain serum; AMBS, rabbit anti‐mouse brain serum; BM, bone marrow; CFU‐C, colony forming unit in vitro; CFU‐S, spleen colony forming unit; CSF, colony stimulating factor; FCS, fetal calf serum; MEM, minimal essential medium; NRS, normal rabbit serum; PBS, 0.01 M sodium phosphate, 0.15 M sodium chloride, pH 7.4.fragments were as effective as the intact IgG in decreasing colony formation. Fab fragments were not suppressive. These results suggest that colony formation is induced via a dynamic interaction between CSF and the progenitor cell membrane, and that antibody directed at cell membrane antigen(s) interferes with the generation of the ind