A radioimmunoassay is described that allows rapid determination of a urinary oligosaccharide — Glcα1-6Glcα1-4Glcα1-4Glc [(Glc) 4] — at concentrations >2 pmol/μl. Antibodies produced in rabbits immunized with the phenethylamine derivative of (Glc) 4 coupled to keyhole limpet hemocyanin bind tritiated (Glc) 4-alditol. Studies comparing the activities of (Glc) 4 and several of its derivatives and analogues as inhibitors of binding of tritiated (Glc) 4-alditol show that (Glc) 4 is detected with maximum specificity and sensitivity only after reduction to (Glc) 4-alditol. Quantitation of (Glc) 4 in urine samples reduced with sodium borohydride gives excellent agreement with a previously used but more laborious method that employs gas-liquid chromatography-mass spectrometry (GLC/MS) (correlation coefficient = 0.98). Studies of the excretion rate of (Glc) 4 in urine of women during pregnancy using the radioimmunoassay method confirm and extend previous results using the GLC/MS method.