Recent studies have shown that acyl-glucuronide conjugates are chemically reactive electrophilic metabolites that can undergo transacylation reactions resulting in intra-molecular rearrangement, hydrolysis and covalent binding of aglycone to albumin both in vitro and in vivo. The hypolipidaemic agent clofibrate is eliminated almost entirely as clofibric acid glucuronide in humans and rats. The formation of clofibric acid-protein adducts was investigated in 14 patients receiving 0.5-2.0 g day of clofibrate for hypercholesterolaemia, and in liver homogenates from 20 rats administered 280 mg/kg/day of clofibric acid for up to 21 days. Total clofibric acid concentrations in the patients ranged from 0 to 114 mg L. Covalently bound clofibric acid-protein adducts were detected in all patients, even in one subject in whom there was no measurable plasma clofibric acid. Concentrations ranged from 2.2 to 53.4 ng mg protein and, in eight patients receiving 1.0 g day of clofibrate, were correlated (P < 0.05) with renal function as assessed by creatinine clearance. Clofibric acid-protein adducts were also present in rat liver homogenates, and increased with increasing duration of treatment (P < 0.0001), from a mean (SE) of 10.1 (0.7) to 32.3 (1.6) ng mg protein. The covalent binding of drugs to tissue macromolecules has traditionally been associated with toxicity. Further research is required to elucidate the role of acylglucuronide conjugates in the formation of drug-protein adducts and their biological consequences.