A clearabce patwayy common to many aryl alkanoic acids is the generation of renally eliminated ester glucuronides. These metabolites are susceptible to systemic hydrolysis which generates the parent aglycone. We have conducted in vitro studies with biosynthetic R- and S-ketoprofen glucuronides to elucidate the mechanism of this phenomenon. These conjugates were incubated in human plasma, various concentrations of human serum albumin (HSA) and protein-free buffer. It was apparent that albumin, rather than plasma esterases, catalysed the hydrolysis of the glucuronides. The albumin-catalysed hydrolysis of ketoprofen glucuronides was highly stereoselective. The mean (±SD) hydrolysis half-life of R-ketoprofen glucuronide in plasma (N = 4) at physiological pH and temperature was 1.37 (±0.30) hr. The corresponding value for S-ketoprofen glucuronide, 3.46 (±0.84) hr, was significantly different (P < 0.005). In contrast, synthetic ethyl esters of R- and S-ketoprofen were hydrolysed by plasma esterases, but not by HSA, and with little stereoselectivity. The reversible protein binding of ketoprofen glucuronides was determined at physiological pH and temperature by a rapid ultra-filtration method. The binding of R- and S-ketoprofen glucuronide to human plasma protein was independent of concentration (P > 0.05) over the range of 1-20 μg/mL. The mean (±SD) percentage unbound in plasma (N = 4) of R-ketoprofen glucuronide was 12.6 (±1.4) %. The corresponding value for S-ketoprofen glucuronide, 9.12 (±0.54) %, was significantly different (P < 0.005). S-Ketoprofen glucuronide was also more avidly protein bound in physiological concentrations of HSA. However, this stereoselectivity decreased in more dilute HSA solutions. Based on the hydrolysis and protein binding data for ketoprofen glucuronides, we propose the existence of separate binding and catalytic sites on the albumin molecule for these metabolites.