We used hybridoma-derived monoclonal antibody to H-2Kk antigens and xenoantibodies to beta 2-microglobulin (beta 2 m) to study the expression of the H-2 antigenic molecular complex on murine hematopoietic stem cells and the effect of long-term culture in vitro on the expression of these antigens. Monoclonal anti-H2Kk antibody produced potent complement-dependent inhibition of pluripotent (CFU-S), myeloid (CFU-C), and erythroid (CFU-E) stem cells from the bone marrow of C3H and AKR(H-2k) mice and was without effect on stem cells from Balb/C (H-2d) mice. Anti-beta 2m xenoantibodies inhibited stem cells from all three strains. Stem cells could not be 'rescued' from the inhibitory effects of the antibodies by the addition of thymocytes to marrow cells after antibody treatment. Both the anti-H-2Kk monoclonal antibody and the anti-beta 2m xenoantibodies produced potent inhibition of AKR (H-2k) CFU-C that had been maintained in culture for up to 6 weeks. These results indicate that murine CFU-S, CFU-C, and CFU-E express H-2 antigens and that the expression of these antigens by CFU-C is not altered during long-term culture.