Antisera were raised in rabbits against histamine conjugated to human serum albumin (HSA) by the carbodiimide (ECDI) method. The specificity of the antisera was studied in a radioimmunoassay using 125I-protein A for detection of IgG binding. The HIS-HSA antisera reacted with histamine-HSA conjugates prepared by either the carbodiimide or diisocyanate coupling procedure, as well as with carbodiimide-prepared histamine-ferritin and histamine-ovalbumin conjugates. On the contrary, the antisera were unreactive with unconjugated HSA, ECDI-reacted HSA, or HSA conjugated to ethanolamine or pentylamine. Free unconjugated histamine significantly inhibited antibody binding to histamine-HSA and 50% inhibition of antibody binding (IC50) was recorded at 3 mM histamine concn. On a histamine molar concn basis a much lower inhibitory potency of free histamine was recorded, as compared to histamine-protein conjugates (IC50 = 3 X 10(-6) mM). This probably reflected amplification of antibody binding to the multivalent ligand, but possibly also that the protein carrier adds some common features to the antigenic determinant. Histidine, ornithine, glutamine, asparagine, sterylamine and several other amino acids lacked inhibitory effects. Histamine H1 and H2 receptor antagonists inhibited histamine binding to the histamine antibodies. The antagonists varied in their affinity for the histamine antibodies and 50% inhibition of antibody binding was recorded in the range of 1-50 mM concn of the antagonists. Comparing one H1 and one H2 antagonist (diphenhydramine and cimetidine, respectively) two of the sera were preferentially inhibited by cimetidine whereas the third serum seemed to be more prone to inhibition by diphenhydramine.