Exposure of human bone marrow mononuclear cells to hepatitis B virus results in the suppression of the in vitro growth of several hematopoietic progenitor cells. We studied the degree of inhibition of erythroid progenitor cells that results as a function of the time of exposure of mononuclear cells to hepatitis B virus and the ratio of virus to mononuclear cells, the multiplicity of infection. With an overnight incubation of mononuclear cells with hepatitis B virus-containing sera, a multiplicity of infection of greater than one virus per mononuclear cell is required to observe significant inhibition of erythroid colony formation. This multiplicity of infection effect is also observed with purified Dane particles. Exposure of mononuclear cells to an increasing number of Dane particles results in a dose-dependent suppression of erythroid colony formation with significant inhibition observed with a multiplicity of infection of virus to mononuclear cells as low as 5:1. Murine monoclonal antibodies to HBsAg completely neutralize the hepatitis B virus-mediated inhibition of CFU-E while control antibodies do not. Purified HBsAg has no effect on colony formation. In conclusion, the hepatitis B virus-mediated inhibition of erythrogenesis in vitro provides a basis for understanding the bone marrow depression observed during hepatitis B virus infections and may provide an in vitro model for examining hepatitis B virus infection.