Infection of mouse BALB/c 3T3 cells by adenovirus 5 resulted in at least 1000-fold lowered yields of virus compared to human cells. The molecular basis of this restriction was analysed at the level of viral gene expression. Steady-state levels of viral DNA and RNA were greatly reduced in infected mouse, compared to human cells. Both early region 1A (E1A) and E1B mRNAs were decreased in mouse cells and their protein products were barely detectable by metabolic labelling of infected cells. The E2A-72 kDa protein and the hexon protein were detected by metabolic labelling, and immunocytochemical analysis showed that they were correctly located in nuclei of infected mouse cells. Only a minor proportion of infected mouse 3T3 cells expressed the E2A-72 kDa or hexon proteins. Low yields of virus were obtained by infection of SV40 transformed BALB/c 3T3 cells showing that SV40 does not provide a helper function for adenovirus 5 growth in this cell system.