The hydrolysis of egg yolk lecithin was studied as catalysed by porcine pancreatic phospholipase A(2) (phosphatide 2-acyl-hydrolase, EC 3.1.1.4) entrapped in reversed micelles of lecithin and sodium dioctyl sulphosuccinate (AOT) in isooctane. The influence of relevant parameters such as temperature, pH, water content, and buffer, AOT, lecithin, calcium and enzyme concentrations was investigated. The order in which the reactants were mixed showed a strong influence on catalytic activity. Higher activities were obtained if the enzyme, water and calcium were first solubilized and pre-equilibrated in plain AOT reversed micelles before the addition of the substrate. Maximum activity was obtained at 750 mmol dm(-3) calcium in the water pool of the micelles, which is an extremely high concentration when compared with the values reported in the literature. This was related to the formation of divalent chelates of calcium with AOT that compete with the binding of the co-factor to the enzyme. Accordingly, the optimum concentration of calcium in the inner core of the micelles decreased from 750 to 500 mmol dm(-3) when the AOT concentration was lowered from 15 to 10 mmol dm(-3).