Histamine receptors have been demonstrated on lymphocyte membranes by a variety of techniques. A method that allows for the flow cytometric analysis of histamine receptors on human peripheral T cells is reported. Histamine is conjugated to fluoresceinated human albumin by the coupling agent ECDI [1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride]. This conjugated histamine compound (FHA-his) binds to approximately 45% of T cells. Fluoresceinated human albumin alone (FHA), not conjugated to histamine, does not bind to T cells. Unconjugated histamine can inhibit completely the binding seen with FHA-his. This technique apparently demonstrates specific FHA-his binding to histamine receptors on T cells and can be used to determine the number of cells bearing such receptors. The reagent could be used with a cell sorter to isolate distinct histamine receptor-bearing (HR+) cells for further immunologic study.