Isolated plasma membranes and naturally shed plasma membrane vesicles from cultured human breast cancer cells were examined by electron microscopy and enzyme analysis. Alkaline phosphatase activity and cytotoxic antibody binding were increased in vesicles as compared to membranes. The activities of gamma-glutamyl transpeptidase (GGTP) and protein kinase were high in membranes and low or absent in vesicles. The observed differences between the plasma membrane and the naturally shed plasma membrane vesicles suggest that the vesicles are micromaps of selected parts of the plasma membrane. These membrane micromaps may allow the tumor cell to avoid destruction by the host's immune system.