The effect of IFN-α and ribavirin alone and combined IFN-α and ribavirin treatment on the proliferation of the murine and human RCC cells. (A) Suppression of proliferation induced by IFN-α alone or in combination with ribavirin. (B) Suppression of proliferation induced by ribavirin alone or in combination with IFN-α. C1: combination of 500 IU/ml of IFN-α and 500 μM ribavirin. C2: 1000 IU/ml of IFN-α and 1000 μM ribavirin. The suppression was significant in the range of 20–1000 IU/ml of IFN-α or 100–1000 μM ribavirin. (*indicates a significant difference in comparison with controls).