Coated vesicles may be an important component of the micropinocytic system of the human placenta. Regions of very dense reaction with glycocalyx stains are restricted to membranes with forming and fully formed coated vesicles. This is interpreted as evidence against permanently grouped specific binding sites having a role in the selective uptake of materials by micropinocytosis, and as a support for theories of coated-vesicle formation which take into account the dynamic nature of membrane components. The pyroantimonate precipitation technique which was employed in an attempt to localize cations in placental tissue at term resulted in the deposition of electron-dense material in coated vesicles and basement membrane. Probably more than 89% of coated vesicles lie within the largest vesicles' diameter from the cell surface. Placental coated vesicles were isolated and examined using negative staining. A polygonally patterned structure was apparent on their surfaces. Analysis of the isolated fraction of coated vesicles using sodium dodecyl sulphate polyacrylamide gel electrophoresis shows the presence of a major protein of molecular weight 180000. This is the same molecular weight that has been given for clathrin, the major protein of the raised polygonally patterned structure on the cytoplasmic surface of coated vesicles from other sources.