The development of a radioimmunoassay (RIA) for the sub-ng ml-1 determination of alosetron, a potent and selective 5HT3 receptor antagonist, in human urine and saliva is described. The antiserum was raised in Soay sheep following primary and booster immunizations with an immunogen prepared by conjugating alosetron-p-azobenzoic acid to bovine serum albumin (BSA). The radioligand consisted of alosetron specifically 125-iodinated on the 2-position of the imidazole group. The mean (+/- standard deviation) theoretical sensitivity (minimum detectable dose corresponding to the imprecision of the zero standard) of the RIA is 3.2 +/- 2.6 pg ml-1 (n = 12) of alosetron in assay diluent (0.1% m/v gelatine-0.05% m/v sodium azide in 0.1 mol l-1 phosphate buffer solution, pH 7.4). The working calibration range using 0.1 ml samples of saliva and 20-fold diluted urine is 0.10-6.40 ng ml-1 of alosetron. Urine samples were diluted prior to assay to overcome adverse matrix effects; consequently, the lower limit of quantification for undiluted urine is 2.0 ng ml-1 of alosetron. Inter- and intra-assay bias and imprecision over the working calibration range were generally < +/- 12% and < 13%, respectively, except at the 0.10 ng ml-1 alosetron level, where the corresponding values were < +/- 17.3% and < 20.2%. The antiserum was free from adverse cross-reactivity with either a synthetic precursor of alosetron or with four major metabolites of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)