The expression of histamine receptors on the surface of rat lymph node cells was studied using a reagent made by directly coupling fluorescein isothiocyanate (FITC) to histamine. This approach contrasts with the use of previous reagents, made by coupling histamine and fluorescein separately to a protein carrier, which bind non-specifically to cells and cause staining unrelated to histamine receptor expression. The new reagent was used, in combination with a panel of monoclonal antibodies, for the dual staining of rat lymph node cells for two-colour flow cytometric analysis to investigate the distribution of histamine receptors on different leucocyte subsets. The majority of cells were stained by the FITC-histamine reagent and these constituted two distinct populations, those with the properties of small lymphocytes and a second population which included macrophages. Inhibition studies with the drugs mepyramine and cimetidine, which are antagonists of H1 and H2 receptors, respectively, showed that most lymphocytes possess H1 receptors while the macrophages have H2 receptors. It seems that macrophages have a higher number of histamine receptors than the majority of lymphocytes, but that they are of lower affinity.