The interactions between glucose oxidase and phospholipid vesicles were investigated. The investigations were carried on molecules adsorbed on the outer surfaces as well as entrapped in the interior of the vesicles . The adsorption of glucose oxidase on the surfaces of egg egg licithin vesicles, containing varying amounts of cholesterol and stearoylamine was measured by determining the free fraction of glucose oxidase detected in the filtrates. In general an enhancement of enzymic activity was observed upon interaction with the vesicles. The enhancement depends on the lipid composition of the vesicles and the surface concentration of the adsorbed glucose oxidase. It reached a maximal value at a surface concentration of 1.4-10(11) molecules/cm2 (approximately 7.1 - 10(4) A2/molecule) on pure phosphatidylcholine vesicles and about 6.5 - 10(10) molecules/cm2 (approximately 16 - 10(4) A2/molecule) when the vesicles contained cholesterol or cholesterol and stearoylamine. CD measurements indicated that the change in enzymic activity of the adsorbed glucose oxidase was accompanied by conformational modification of the enzyme. In order to entrap glucose oxidase into the vesicles, the lipid was sonicated in the presence of the enzyme. After removal of the free and adsorbed enzyme the amount of the entrapped enzyme was determined by measuring its activity after disintegration of the vesicles with Triton. The enzymic activity of the entrapped glucose oxidase served as a measure for the permeability of the bilayer membrane of the lipid vesicles to glucose. Addition of insulin to the suspension of vesicles containing the entrapped glucose oxidase increased the permeability of glucose by up to 9 - 10(-8) cm/s. This value is the lowest estimate based on the assumption that one glucose oxidase molecule was entrapped in every vesicle.