Cellular interactions responsible for regulating in vitro erythropoiesis were studied using murine monoclonal antibodies recognizing antigens expressed by human mononuclear cells. Cell populations of interest were negatively selected by complement-dependent cytotoxicity and then evaluated for their effect on in vitro growth of erythroid burst-forming units (BFU-E). Normal peripheral blood T cells apparently contain at least 2 functionally distinct subpopulations with opposing regulatory effects: one that enhances burst formation and one that limits burst formation. Whether these effects are mediated by direct interactions of T cells with BFU-E or with auxillary cells remains to be determined.