Expression of major histocompatibility complex (MHC) class II molecules is developmentally regulated and lineage dependent. Their role in hematopoiesis is not well defined. Previous studies in a canine model showed that dogs given 920 cGy of total body irradiation, transplanted with autologous marrow, and treated with anti-MHC class II monoclonal antibody (MoAb) immediately posttransplant experienced only a transient granulocyte recovery that was followed by graft failure. In the present study, the effect of anti-MHC class II MoAbs on canine in vitro hematopoiesis was investigated. Anti-MHC class II MoAb H81.9 or B1F6 (both recognizing nonpolymorphic determinants) had no inhibitory effect when added directly to colony-forming unit-granulocyte-macrophage (CFU- GM) grown in agar. However, the addition of intact MoAb or as F(ab')2 fragments to long-term marrow cultures (LTMCs) resulted in a dose- dependent inhibition of the generation of CFU-GM among nonadherent cells. Inhibition was most profound with MoAb added at the time of initiation of culture. However, even if MoAb was added 3 weeks after recharging LTMCs, CFU-GM generation rapidly decreased. In addition, the number of adherent cells in LTMCs decreased; predominantly fibroblast- like cells with prominent cytoplasmic vesiculation remained. Acridine orange/ethidium bromide staining and TdT-mediated deoxyuridine triphosphate-digoxigenin nick end labeling (TUNEL) tests showed an increase in the proportion of apoptotic cells in both the nonadherent and adherent compartments. Binding of anti-MHC class II MoAb to unfractionated marrow cells resulted in an increase in free (Ca2+)i; no changes in tyrosine phosphorylation pattern were observed. The addition of stem cell factor (SCF), but not granulocyte colony-stimulating factor or granulocyte-macrophage colony-stimulating factor, to LTMCs prevented apoptosis, and the generation of CFU-GM was indistinguishable from controls. Similarly, a supportive adherent layer was maintained. Thus, anti-MHC class II MoAbs interfere with hematopoiesis both in vitro and in vivo. The mechanism involves programmed cell death in subpopulations of adherent and nonadherent cells. Inhibition of hematopoiesis is abrogated by exogenous SCF.