Ultrastructural studies of the uptake of enveloped and naked frog virus 3 (FV 3) particles by BHK-21 cells have shown that enveloped viruses are internalized by adsorptive endocytosis via coated pits. The enveloped particles then appear to move through endosomes and finally lysosomes. Naked viruses may also follow the same pathway but only rarely. Their more frequent mode of entry is by fusion between the virus shell and the cellular membranes, thus allowing the virus to shed its nucleoprotein content directly into the cytoplasm. This difference in the mechanism of penetration has been confirmed by the use of lysosomotropic agents: the inhibition of viral growth being far more drastic for enveloped FV 3 than for naked virus implies that a lysosomal step is required for the multiplication of enveloped viral particles.