A method for assaying mono(2-ethylhexyl)phthalate (MEHP) uridine diphosphate (UDP) glucuronyl transferase activity in microsomal preparations from guinea pig liver is described. The quantitation of the MEHP-glucuronide was performed by HPLC after direct injection of a sample of deproteinized incubation mixture. Solubilization of microsomal UDP-glucuronyltransferase activity was achieved by use of Lubrol, and optimal conditions for glucuronidation of MEHP were established. To investigate whether there is competition between MEHP and bilirubin for glucuronidation, inhibition experiments were performed with solubilized enzyme preparations. In these incubations addition of bilirubin decreased the formation of MEHP-glucuronide. No change in the maximal conversion rate (Vmax) was observed, indicating the occurrence of competitive inhibition. This observation may have implications in clinical situations where patients with hyperbilirubinemia are exposed to MEHP, e.g. in exchange transfusions in newborn infants.