Using the principle of relief of self-quenching of carboxyfluorescein [Weinstein, J. N., Yoshikami, S., Henkart, P., Blumenthal, R., & Hagins, W. A. (1977) Science 195, 489-492] upon leakage of the dye from the interior of lipid vesicles, we investigated the integrity of sonicated small unilamellar vesicles in the presence of isolated hepatocytes, Zajdela ascites hepatoma cells, and plasma membranes of either cell type. We observed that cells as well as plasma membranes induce leakage of carboxyfluorescein from vesicles. Two parameters (initial rate and maximal level of induced leakage) were determined to quantitate the leakage events and were found to depend on cell density, vesicle concentration, and vesicle lipid composition. The magnitude of both parameters is shown to increase with cell density and to decrease with increasing vesicle lipid concentration and seems to be proportional to the number of vesicles found in close contact with the cell. For vesicles made of phosphatidylcholine and cholesterol, the degree of induced leakage increases steeply with cholesterol contents increasing from 30 to 40 mol %. In the case of simultaneous presence of 10 mol % phosphatidylserine, induced leakage can be observed at cholesterol contents exceeding 20 mol %. We show that leak-inducing activity resides in the plasma membrane and that it can be considerably reduced by treatment of the plasma membranes with neuraminidase or trypsin, suggesting the involvement of cell-surface glycoprotein(s). Release of activity from intact cells and isolated plasma membranes into the medium occurs spontaneously (at a slow rate) but can be facilitated by freezing and thawing; the activity can subsequently be recovered in a soluble form from the medium.