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 < 0.005). When donors either positive or indeterminate by RIBA II were compared with donors negative for hepatitis C antibodies, the odds ratio for a possible parenteral source of infection was 7.6 (p = 0.0000). Subjects who had received a poor education (odds ratio 0.3, p < 0.001) or who lived in southern Finland (odds ratio 2.3, p < 0.05) were also at higher risk for being positive or indeterminate in RIBA II. First-time donors were also prone to having antibodies according to RIBA II (odds ratio 2.2, p = 0.1), whereas sexual risk behavior, gender, age, occupational class and type of residential area were not risk factors for hepatitis C antibodies in RIBA.(ABSTRACT TRUNCATED AT 250 WORDS)