Synthesis of the adenovirus type 12 E1A and E1B tumour antigens was compared in productively infected human (KB) cells and in abortively infected baby hamster kidney (BHK) cells. By the use of anti-peptide antibodies, the E1A tumour antigens were easily detectable in infected KB extracts as early as 6 h post-infection, but were not detectable in infected BHK extracts until 12 h post-infection. The level of E1A tumour antigens detected in BHK extracts was 10 to 15% of that detected in KB extracts. The level of the E1B 163R (19K) tumour antigen was also lower in BHK extracts: 5 to 10% of that detected in KB extracts. Stability of the E1A tumour antigens was not significantly different in the two infected cell species, indicating that the lower E1A level during abortive infection was due to a lower rate of synthesis of these proteins. These data suggest that early protein synthesis is not the same in abortively infected cells as it is in productively infected cells.