In situ coating of the surface of endothelial cells in rat lung with cationic colloidal silica particles was used to separate caveolae from detergent-insoluble membranes rich in glycosyl phosphatidylinositol (GPI)-anchored proteins but devoid of caveolin. Immunogold electron microscopy showed that ganglioside G(M1)-enriched caveolae associated with an annular plasmalemmal domain enriched in GPI-anchored proteins. The purified caveolae contained molecular components required for regulated transport, including various lipid-anchored signaling molecules. Such specialized distinct microdomains may exist separately or together in the plasma membrane to organize signaling molecules and to process surface-bound ligands differentially.