Demographic variables, sexual risk behavior and prevalence of parenteral risk factors were studied in 305 randomly selected donors seronegative for hepatitis C virus, in 170 randomly selected donors reactive on solely enzyme-linked immunosorbent assay (ELISA C-100), in 71 consecutive donors reacting indeterminately according to the second-generation recombinant immunoblot assay (RIBA II) and in 46 consecutive donors found to be positive using the RIBA II. Donors who were positive by RIBA II had significantly more often a risk factor, for example use of intravenous drugs or previous blood transfusion, than donors reacting indeterminately (34 out of 46) (73.9%) versus 14 out of 71 (19.7%, p = 0.0000). Donors reacting indeterminately by RIBA II had one of those risk factors significantly more often than seronegative donors (14 out of 71) (19.7%) versus 23 out of 280 (7.8%, p