We screened 74 patients with nonA nonB acute hepatitis (37 of post-transfusion PTH, and 37 of non-post-transfusion, NPTH, origin) for the presence of anti-HCV by tests detecting either C100-3 antibodies alone (ELISA I) or C100-3 plus C33c plus C22-3 antibodies (ELISA II). Samples were taken at the onset of disease and then serially for a period of time ranging from 12 to 60 months. An increased number of anti-HCV positive cases (86% vs 69%) and an earlier seroconversion were observed with the second compared to the first generation ELISA. Positive samples were confirmed by recombinant immunoblot assay (RIBA), which was also used to study the kinetics of the antibody response to individual HCV antigens. Anti-C33c and anti-C22-3 antibodies were the first detectable markers of HCV infection in 80% and 20% of the patients, respectively. Thirty-two percent of the patients studied showed complete and persistent biochemical recovery, whereas 68% maintained a chronic elevation of the transaminase values. Among the 20 patients who showed early and persistent normalization of the transaminase values, complete disappearance of all antibody reactivities was limited to five of them, whereas in four cases only anti-C100-3 and anti-5-1-1 became negative.