For assessing the role of circulating immune complexes (CIC) in chronic hepatitis C, the relative frequency of CIC was determined in 54 patients with chronic hepatitis C, 15 asymptomatic hepatitis C virus (HCV) carriers, and 54 healthy controls. IgM and IgG containing CIC were studied using both C1q and conglutinin (K) in an immunoglobulin-specific solid-phase enzyme immunoassay. CIC were a common feature of chronic hepatitis C with 96.3% of patients with at least one abnormal test result. The prevalence of elevated IgG-K, IgM-K, IgG-C1q, and IgM-C1q CIC was 70.3%, 50.0%, 64.8%, and 35.1%, respectively. The prevalence of IgG class CIC was higher than IgM class CIC (P = 0.038 for K-CIC and P = 0.01 for C1q-CIC, respectively). There is correlation between IgG-K CIC and IgG-C1q CIC (r = 0.445, P = 0.002), IgG-K CIC and IgM-C1q CIC (r = 0.348, P = 0.020), IgM-K CIC and aspartic aminotransferase (r = 0.321, P = 0.015), IgM-K CIC and alanine aminotransferase (r = 0.301, P = 0.027). Compared to patients with chronic persistent hepatitis and chronic lobular hepatitis, patients with chronic active hepatitis have a higher prevalence of elevated IgG-K CIC (77.2% vs. 40.0%, P = 0.029) and IgM-K CIC (56.8% vs. 20.0%, P = 0.038). The concentration of IgG-K, IgM-K, and IgM-C1q CIC in the former was significantly higher than that in the latter, respectively. In conclusion, IgG class CIC is the major type of CIC in chronic hepatitis C.(ABSTRACT TRUNCATED AT 250 WORDS)