In order to investigate the heterogeneity of clathrin-coated vesicles purified from rat liver, and to quantitate rigorously their membrane contents, we have analyzed scanning transmission electron micrographs of unstained coated vesicles before and after extraction with the non-ionic detergent Triton X-100, as well as of vesicles whose coats had been removed by dialysis against 10 mM or 100 mM Tris (pH 8.2). Their respective distributions of particle masses were thus determined and compared, in light of complementary biochemical quantitations of lipid and protein. Smaller coated particles, 25-45 MDa in mass and 60-80 nm in diameter, lose no mass when extracted with Triton, and disappear when their coats are dissociated. We conclude that they do not contain membrane vesicles, although they have dense, presumably proteinaceous, cores. They may represent particles generated during tissue homogenization or, possibly, a storage form of clathrin. The remaining 70% contain bona fide vesicles: these particles are 75-150 nm in diameter, and their average mass is about 80 MDa, of which 48 MDa is contributed by coat proteins, 10-12 MDa by phospholipid and cholesterol, and 20-22 MDa by vesicle-associated proteins. Their vesicles are of two types: smaller, denser, vesicles that contain substantial amounts of internalized material, and larger, less dense, vesicles that do not. The distinction between them may, in view of other findings, reflect a difference between coated vesicles derived respectively from the Golgi and the plasma membrane.