Budding of frog virus 3 (FV3) from the cytoplasmic membrane of BHK21 cells has been studied with immunological and cytochemical methods in electron microscopy. In stage 0, where the virion is close to the cellular membrane, no modifications were observed. Stage 1 is characterized by the contact between the virion and the cell membrane. In stage 2, the virus is bound to the cell by a peduncle, whereas in stage 3 it has been released. The viral envelope is composed of an outer trilaminar unit which is the continuation of the cytoplasmic membrane and of an inner layer of electron-dense material. Virus-specific antigens as well as cytochemical modifications are strictly localized in the areas of the membrane where the virions are budding. Enveloped particles keep a cellular specificity.