In order to evaluate the potential role of cortical vesicle exocytosis in modifying the egg surface at fertilization, we have begun characterization of the cortical secretory vesicle. Earlier reports (N. K. Detering, G. L. Decker, E. D. Schmell, and W. Lennarz, 1977, J. Cell Biol. 75, 899-914) have described the isolation of an egg cell surface complex which consists of the egg plasma membrane and cortical secretory vesicles. We have now developed a method of dissassembling the cell surface complex and isolating the cortical vesicles. The very low levels of contaminating plasma membrane in this preparation allow the meaningful comparison of plasma membrane and cortical vesicle proteins and lipids. The cortical vesicles were found to be rich in high-molecular-weight PAS-positive proteins. The majority of these glycoconjugates were solubilized by hypotonic lysis of the cortical vesicles and probably represent proteins sequestered inside the intact vesicles. The fatty acid composition of the cortical vesicles was found to be unusually high in arachidonic acid. The fatty acid composition of the cortical vesicles was closely similar to that of the plasma membrane; however, the cortical vesicles were substantially higher in cholesterol content.