A radioimmunoassay is described that allows rapid determination of a urinary oligosaccharide -- Glc alpha 1-6Glc alpha 1-4Glc alpha 1-4Glc [(Glc)4] -- at concentrations greater than 2 pmol/microliter. 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 by 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.