The improved clinical outcome observed among patients with hepatitis C treated with the combination of alpha interferon (IFN) and ribavirin (RBV) is presumed to result from immunomodulation and viral inhibition. However, the impact of the drug combination upon lymphocyte activity is unknown. The present study evaluated the effects of IFN and RBV, singly and in combination, upon proliferation, cell cycle sensitivity and cytokine elaboration following PHA stimulation of lymphocytes. Two formulations of IFN, interferon-a-2b (IFN-2b) and interferon-a-con-1 (CIFN), were included. Titration of each drug over a wide range of concentrations showed dose dependent proliferative suppression without cytotoxicity. Proliferation was suppressed 57-99% (P<0.001) by IFN-2b (10(5)-10(7) IU/ml), 41-74% (P<0.001) by CIFN (1.5-150 ng/ml), and 10-94% (P<0.001) by RBV (0.5-50 microg/ml). Isobologram analysis showed that the interaction between IFN-2b and RBV on proliferative suppression was additive. In contrast, the interaction between CIFN and RBV was weakly antagonistic. Proliferative suppression by both the IFNs was cell cycle restricted. IFN-2b and CIFN added at the onset of PHA stimulation (G0/G1) versus 24 h later (S phase) inhibited proliferation by 50 versus 5%, respectively (P<0.05). The onset of IFN resistance correlated with a 50% reduction (P<0.05) in IFN receptors on the cell surface. In contrast, RBV caused equivalent proliferative suppression (P=NS) when added at any time during PHA activation. Cytokine secretion after 24 h of PHA stimulation showed that IFN-2b versus CIFN increased the secretion of IL2, TNF and gamma IFN by 4.5-, 4.1- and 8.3-fold (P<0.005) versus 1-, 1.9- and 1.9-fold (P<0.05), respectively, above control levels. Neither IFN affected IL10 secretion. RBV, singly and in combination with IFN, had no impact on cytokine expression (P=NS). This study identifies several potential mechanisms by which the combination of IFN and RBV may exert a more potent effect upon cellular immunity than either agent alone and shows that different formulations of IFN may have non-identical effects upon lymphocyte responses.