The susceptibilities of C57Bl/6J mouse embryo fibroblasts (MEF) and baby mouse kidney (BMK) cells to infection or transformation by adenovirus type 2 have been compared to those of rat embryo fibroblasts (REF). Both MEF and BMK cells were some 10-fold less permissive to replication of the virus than were REF cells, even though similar fractions of all three cell types, a maximum of 50-60%, produced viral tumor and structural antigens. This observation suggests that a very late step in adenovirus production, such as assembly or maturation, occurs much less efficiently in mouse cells than it does in rat cells. No significant differences in the frequencies of transformation, as assayed by the appearance of foci of morphologically transformed cells, were observed following transfection of adenovirus type 2 or type 5 DNA into the three cell types. However, it proved extremely difficult to establish permanent lines of adenovirus-transformed mouse cells: only 2 of more than 100 attempts were successful, compared to a success rate of close to 100% with adenovirus type 2-transformed REF or SV40-transformed MEF or BMK cells. The two lines of type 2 adenovirus-transformed MEF that were established have been shown to retain and express viral genetic information.