A specific and sensitive radioimmunoassay (RIA) for the histamine H2-antagonist, tiotidine, has been developed. The assay is based upon competition of tiotidine with [3H]-tiotidine for antibody (Ab) obtained from immunized rabbits. The immunogen used was a glutaraldehyde coupled conjugate of the tiotidine derivative (ICI 147,655) and bovine serum albumin (BSA). Displacement of [3H]-tiotidine by unlabeled tiotidine was competitive over a concentration range of 10-1000 fmol. The Ab was 100-fold less sensitive to the pharmacologically inactive metabolite of tiotidine (ICI 129,585) and did not cross-react with histamine, cimetidine or phenylguanidine. In dogs given an i.v dose of tiotidine, plasma levels of the antagonist, as measured by the RIA, were correlated with inhibition of histamine-induced gastric acid secretion. Tiotidine (0.3 or 0.6 mumol kg-1) caused a dose-dependent and transient decrease in acid secretion at plasma concentrations of 10(-6) to 10(-8) M. Other potential analytical and research uses of H2-antagonist radioimmunoassays are discussed.