The serologic reactivity and epidemiology associated with different hepatitis C virus (HCV) variants were investigated in a cohort of 113 anti-HCV-positive donors. In Scotland, HCV type 1 accounted for one- half of all infections; 40 percent of subjects were infected with HCV type 3, and the remainder were infected with type 2. Reactivity with the NS-4-encoded antigens in the first-generation anti-c100 assay was absent in 68 percent of donors infected with types 2 and 3, as compared with 10 percent for those infected with type 1. Even when combined with surrogate marker testing, first-generation tests would have failed to detect 12 percent of HCV-infected blood donors. The age distribution, incidence of past infection with hepatitis B virus, and reported risk factors were similar in donors infected with types 1 and 3 (mean ages were 31.9 and 29.9; 18 and 17.5% were positive for antibody to hepatitis B core antigen; and 47 and 48% had past intravenous drug abuse). However, the distributions of alanine aminotransferase levels were significantly different in those infected with type 3 (abnormally raised in 83%) and those infected with type 1 (55% abnormal alanine aminotransferase; p < 0.05) or type 2 (60%; p < 0.01) and those who were nonviremic (8%; p < 0.0001). These data suggest that HCV type 1 is the most common HCV infection in blood donors and that infection with HCV type 3 may be associated with more severe liver disease, because of more recent infection or because of a greater inherent pathogenicity of type 3 variants.