Adenovirus 2 (Ad2) uncoating was analyzed as the destabilization of virions which renders the parental genome sensitive to DNase treatment. This event demonstrated a strong temperature dependence, and an Arrhenius plot of initial uncoating rates revealed an inflection point at around 16 degrees C. Activation energies of 331 kJ/mol below and 88 kJ/mol above this temperature were obtained for the uncoating process. Penetration of Ad2 through the plasma membrane was completely inhibited by sodium azide, whereas uncoating was only slightly influenced. This indicated that uncoating had already taken place at the outside of the plasma membrane. Incubations of Ad2 with isolated plasma membranes and cell homogenates showed that intact and metabolizing cells were required for uncoating. We further suggest, based on the inhibitory patterns of EDTA, EGTA, dansylcadaverine, and dithiothreitol, that this destabilization of virions follows upon reorganization in the plasma membrane. In the electron microscope the involvement of coated vesicles was shown for the initial uptake of virions, possibly followed by the engagement of acidic vesicles as judged from the effects of lysosomotropic agents on gene expression. The vectorial transport of virions from the plasma membrane to the nucleus was not affected by reagents interfering with the cytoskeletal system. Consequently, we propose that Ad2 virions are internalized by adsorptive endocytosis.