1. Suprofen acyl glucuronide, a major metabolite of suprofen in man, is labile, undergoing acyl migration to isomeric conjugates which are not cleaved by beta-glucuronidase. 2. The pH-dependent degradation of diastereomeric suprofen glucuronides in aqueous buffer increases rapidly near physiological pH with an apparent first-order half life of 1.4 h at pH 7.4. 3. Suprofen glucuronide and its isomeric conjugates are reactive with albumin in a pH-dependent manner which corresponds to the stability of the acyl glucuronide. Several per cent of the conjugates added to albumin in solution become covalently bound. 4. The covalent binding of suprofen equivalents to albumin is greatly enhanced by the addition of either cyanide or cyanoborohydride anion, which supports the presence of an imine in the process of binding. Release of isomeric conjugates by treatment of the albumin adduct with dilute acid also supports covalent binding via an imine. 5. The covalent binding of suprofen to proteins through its reactive acyl glucuronide may be of toxicological importance and relevant to the acute renal toxicity observed for suprofen in man.