Many studies report the presence of platelet-megakaryocyte-associated antigens on human immature and mature megakaryocytes as well as progenitor cells of this lineage. However, studies of purified megakaryocytes or megakaryocyte colonies yield little information on the effect(s) of growth factors in regulating antigenic expression, the kinetics of expression, or the relative content of antigens expressed. We report the detection of human megakaryocyte antigens using a solid-phase radioimmunoassay. The assay is linear, specific, and detects a range of approximately 300-10,000 megakaryocytes per aliquot. Analyses of unstimulated and stimulated suspension cultures indicate that activities known to influence megakaryocyte development in vitro also increase total megakaryocyte antigenic content. Additionally, studies on human erythroleukemia cells show that those cells constitutively express megakaryocyte antigens and increase these antigens when stimulated. Finally, since the specificity of the assay is that of the primary antibody, this procedure is easily extended to analysis of antigenic expression in other lineages.