Tumor cells release intact portions of their plasma membranes in the process of membrane fragment shedding. This released material has been shown to inhibit various synthetic functions of normal cells, which may play an important role in certain patho-physiological events occurring in advanced-stage cancer patients. Our studies on metastatic variants of the murine B16 melanoma, B16-F1 (low incidence of lung colonization) and B16-F10 (high incidence of lung colonization) indicate that the shed membrane fragment material is composed predominantly of vesicles, ranging in size from 20 to 100 nm in diameter. The release of membrane fragments represents a small percentage (approximately 16%) of the total shedding of plasma membrane components. Membrane fragments were shed at a higher rate from the highly "metastatic" (colonizing) B16-F10 cells than from poorly metastatic B16-F1 cells, resulting in a 2-fold greater accumulation of membrane fragment material by cultures of B16-F10 cells than by B16-F1 cultures during the 48-hr assay period. The study of various intracellu ar metabolic processes (protein and RNA synthesis, glycosylation, and generation of ATP) required for the shedding of membrane fragments indicated that the shedding event is only dependent on energy when inhibitors of the above processes are present for 2 hr. Treatment of cells with these inhibitors for 8 hr results in cessation of the shedding process, indicating both a limited pool of components to be shed and the requirement for further synthesis of the shed material. Glycoprotein components of the shed membrane fragments were analyzed by SDS-polyacrylamide gel electrophoresis. In addition to quantitative differences, 2 additional bands were present in fluorographs from SDS-PAGE gels from the B16-F10 membrane fragment material which were not present in fluorographs from B16-F1 fragments. The glycoprotein components of shed membrane fragments were shown to represent selected domains of the cell's plasma membranes, in that only certain plasma membrane glycoproteins are shed as part of membrane fragments. The glycoproteins released as non-particulate molecules into the extracellular environment failed to exhibit these quantitative and qualitative differences.