The effects of ribavirin and interferon (IFN) alpha have been investigated on cultured peripheral blood mononuclear cells, obtained from 15 patients with chronic hepatitis C virus (HCV) infection. At clinically relevant serum concentrations achieved during therapeutic administration, ribavirin did inhibit moderately the mitogen-stimulated mononuclear cell proliferation and growth of the CD4+ and CD8+ T cell subsets without apparent cytolysis. The ribavirin-IFN-alpha combination showed activity against HCV with disappearance of HCV RNA in 27% of cases, and a synergy in the inducibility of the intracellular enzyme 2',5'-oligoadenylate synthetase. Such ribavirin concentrations induced modest increases in the T helper 1-like cytokine production by mononuclear cells. Higher ribavirin concentrations markedly inhibited IFN-gamma production, but augmented interleukins (IL) 2, 4, and 12 secretion. Conversely, IFN-alpha tended to suppress IL 2, 4 and 12, but enhanced IFN-gamma and IL-10 secretion. Thus, ribavirin and IFN-alpha appear to cause diverse effects on immunoregulatory cytokine secretion, and when combined, counteracted for production of IL-2 and IL-12, while upregulated mononuclear cell secretion of IFN-gamma and that of the anti-inflammatory cytokine IL-10. These findings suggest a non-cytolytic modulation of inflammatory responses induced by the drug combination, that may be relevant in the pathophysiology of chronic HCV infection.