A specific and sensitive radioimmunoassay was developed for SR 31747A, a new sigma ligand, using a monoclonal antibody. This antibody was produced from spleen lymphocytes of a mouse immunized with SR 31747A coupled to bovine serum albumin via a peptide bond using SR 120684A, a succinamic acid derivative of SR 31747A. Negligible binding occurred when metabolites, obtained by chemical synthesis or by "in-vitro" incubation with hepatic microsomal fraction, were tested for cross-reactivity. A quantitative recovery in serum of the exogenous analyte was obtained for all the concentrations tested and the quantification limit was found to be 0.25 ng ml-1 of SR 31747 (the non-salified derivative). Intra- and inter-assay relative standard deviations ranged from 6.3-10.9 and from 5.3% to 15.4% respectively. Furthermore, comparison of results from samples which were assayed by radioimmunoassay and gas chromatography demonstrated an excellent correlation (r = 0.984).