Purified adenovirus induced a dose-dependent release of the water-soluble markers calcein and carboxyfluorescein from liposomes. Marker release was strongly dependent on pH, and at temperatures below 5 degrees C, the rate of release showed an optimum at a pH of about 6. This pH dependence parallels disruption of endocytic vesicles by adenovirus and the permeabilization that adenovirus induces on the cell surface. There did not seem to be a striking dependence on the lipid composition of the liposomes. Electron microscopy using a negative stain shows liposomes bound to adenovirus. In some cases, the liposomes were still intact, but many liposomes, which were attached to the vertices of the virus, appeared lysed. These data support the notion that adenovirus, which enters the host cell by receptor-mediated endocytosis, gains access to the cytoplasm by a subsequent pH-dependent disruption of the membrane of the endocytic vesicle.