When human transformed RSa cells were treated simultaneously with potent tumor promoter 12-O-tetradecanoyl-phorbol-13 acetate (TPA) and interferon (IFN) (20-100 IU/ml) at the concentration which suppresses the multiplication of cells slightly, distinct synergistic suppression of cell growth was observed. Synthesis of DNA in these cells was reduced in proportion to the concentration of IFN and it was more strongly reduced by combined treatment with IFN and TPA. When IFN was pretreated with anti-IFN serum, such synergistic effect was not observed. Highly purified IFN (specific activity, 2 X 10(8) U/ml) together with TPA elicited almost the same synergistic effect on RSa cells. Similar concentration of phorbol-12,13-didecanoate (PDD) also produced synergistic effects with IFN on RSa cells. In contrast, phorbol (PHR), which lacks tumor promoting activity, did not produce such synergistic effects even at a concentration of as high as 500 ng/ml. In the case of HeLa cells and IFN-resistant IFr cells, only an additive effect was observed. Effect of TPA on the antiviral action of IFN against VSV was tested and there was almost no influence of TPA on IFN action. In accordance with these results, 2-5A synthetase activity was not induced by TPA treatment and the level of enzyme action did not change significantly by the combined treatment with TPA and IFN.