A radioimmunoassay for the antidiabetic drug 3-(4-[beta-(5-methyl-isoxazol-3-carboxyamido)-ethyl]-benzenesulfonyl)-ureido-piperidine-4-carboxic acid (glisoxepide, I, Pro-Diaban) is described. Antisera were obtained by immunizing rabbits with a piperidine carboxylic acid analogue of glisoxepide coupled to bovine serum albumine. The carboxylic acid analogue coupled to [3H]-1-tyrosine methyl ester was used as the tracer. Antisera with high sensitivity were obtained. With some glisoxepide analogues the specificity of the antisera was investigated. For good binding the isoxazole and the benzene ring as well as the appropriate distance between both rings are essential. The assay can be carried out without extraction of the compound from the serum. Dextran coated charcoal is used for separation of free and bound glisoxepide.