The effect of inhibitors of RNA (actinomycin D, Act. D) and protein synthesis (cycloheximide, CH) on induction of Epstein-Barr virus (EBV) antigens by the tumour promoter TPA and by EBV superinfection has been analysed. The data indicate that (i) concomitant treatment of cells with Act. D and TPA leads to complete suppression of virus antigen induction. Subsequent treatment of the cells with Act. D after prior exposure to TPA results in some virus antigen induction, the amount depending on the time of TPA treatment. (ii) Simultaneous treatment of the cells with TPA and CH blocks antigen expression completely. Removal of the inhibitor results in antigen expression at a comparable rate to that of CH-untreated cells. (iii) If Ch treatment is followed by addition of Act. D, virus antigen induction by TPA is completely inhibited. In contrast, superinfection of the cells with P3HR-1 EBV in the presence of CH for the same period followed by removal of the inhibitor and addition of Act. D leads to virus antigen expression by 3 h after Act. D addition. (iv) Concomitant treatment with CH and TPA followed by addition of either iododeoxyuridine or n-butyric acid results in 'superinduction'. Virtually all cells exhibit EBV-specified antigens. This implies that induction of virus antigens by tumour promoters requires the synthesis of a specific RNA, that this RNA increases in concentration during the induction period and that the same RNA is not required for EBV transcription after exogenous infection.