Summary By means of immunofluorescence, the sequence of accumulation of antigenic components of the adenovirus cycle was studied in a variety of cells of primate and non-primate origin after infection with adenovirus type 12 (A-12). Structural viral antigens were seen only in primate cells, and most clearly in human fetal kidney, following the appearance of the early antigens. Structural viral antigen was found in a smaller proportion of monkey cells than human cells, even though a high proportion of monkey cells elaborated large amounts of the early antigens. In non-primate cells, rabbit, hamster, rat and mouse cells readily produced early antigens after infection with A-12, but few cells with early antigen were demonstrable in chick cells, and none in Earle L cells. Among A-12 transformed cell lines, and A-12 induced hamster tumor lines in culture, considerable variation in the continuous elaboration of early antigens was found, with a rabbit transformed line and some hamster tumors producing moderate amounts of early antigens, other hamster tumor lines less, and none at all demonstrable in the 3 rat transformed lines studied. It was concluded that the adenovirus type CPE is probably associated with early events in the viral cycle, since it appeared in cells incapable of producing structural viral antigens, and that the viral-induced events leading to transformation may similarly be those occurring early in the viral cycle. Production of early antigens is not a prerequisite for maintenance of the abnormal morphology and growth properties of the A-12 transformed cell, but if present is merely a marker of residual viral information.