Radiobinding assays using monoclonal H-Y antibodies were performed on human peripheral blood leukocytes, concanavalin-A-(conA)3stimulated blast cells, and fibroblast primary cell cultures. For peripheral blood leukocytes, male-specific binding was demonstrated only when the cells were enriched for T cells and the H-Y antisera were partially purified for the IgG or IgM fractions. Similar results were obtained with con-Astimulated blast cells. Fibroblast cells could be tested with unpurified H-Y antisera. With our monoclonal antibodies, best results were obtained using con A blast cells and fibroblasts.