The mesocarp of mature oil palm fruit undergoes intensive triglycerides hydrolysis upon abscission and bruising. This generates such a high amount of free fatty acids that the oil might become unfit for human consumption without appropriate refining. The lipase (EC 3.1.1.3) involved in the breakdown of the oil is not stable after homogenization of the tissue in aqueous buffers. In this study, we have devised a solvent-based procedure that allowed us to obtain fractions with stable lipase activity. Using these fractions, we have determined the optimal conditions for assaying mesocarp lipase activity. The activity was highest at a temperature of 35 degrees C and a pH of 9. The lipase was found to be strictly calcium dependent. The specific activity of the lipase measured in optimal conditions was found to be 33 mumol fatty acids released min(-1) mg(-1) protein using olive oil as substrate. The mesocarp contains about 190 U of lipase g(-1) fresh weight. This activity was found to be inhibited by the lipase inhibitor tetrahydrolipstatin (THL), suggesting that the lipase is a serine hydrolase.