Results from a multicentre, clinical trial of interferon-alpha2a (IFN-alpha2a) for the treatment of chronic hepatitis C are reported. Serum hepatitis C virus (HCV) RNA levels were monitored as follows: before, and 2 days after, the first administration of IFN-alpha2a; during and at the end of treatment; and 6 months after completion of therapy. Peripheral blood lymphocyte subpopulations were measured, by two-colour flow cytometry, before and 3 h after the first intramuscular (i.m.) administration of 9 mega units (MU) of IFN-alpha2a. Virological responders had a significantly lower pretreatment level of CD11+ CD8- lymphocytes. Biochemical responders had significantly lower pretreatment levels of CD11- CD8+, human leucocyte antigen (HLA) DR- CD4- and HLA DR- CD8+ populations, and a higher pretreatment HLA DR+ CD4- population. These pretreatment differences disappeared 3 h after the first i.m. administration of IFN-alpha2a. CD11- CD8+ and HLA DR+ CD8+ cell populations became significantly lower in virological responders 3 h after the first i. m. administration of IFN-alpha2a. HLA DR+ CD4+ cell populations were increased less in biochemical responders. Thus, T-lymphocyte subpopulations were different between responders and non-responders to IFN therapy and IFN-modulated host immunity. Multivariate analysis showed that the pretreatment CD11+ CD8- cell population was an independent predictive factor of response to therapy. On the other hand, patients whose serum HCV RNA cleared or decreased within the first 2 days of IFN-alpha2a therapy were more likely to achieve a virological response. This predictive factor, however, was not an independent factor by multivariate analysis. These results suggest that host immunity is an important factor in response to IFN therapy, and HCV clearance within the first 2 days is a good predictive factor of response.