The serological responses to two different hepatitis C virus antigens were studied by enzyme-linked immunosorbent assay in a variety of chronic liver diseases and in healthy blood donors. The study population comprised 97 cases of cryptogenic chronic liver disease (40% with a history suggestive of parenterally transmitted non-A, non-B hepatitis and 60% without such a history), 87 cases of other well-characterized chronic liver diseases and 96 voluntary blood donors. The commercially available C100-3 assay and a new assay utilizing a 22 kD recombinant protein (c22) from the nucleocapsid region of the virus were used for antibody detection. Overall in the non-A, non-B hepatitis group, 77% were positive for anti-c22, 55% were positive for anti-C100-3 and 24% were negative by both tests. In the parenterally transmitted chronic liver disease group, 82% were positive for anti-C100-3 and 90% were positive for anti-c22 (not significant). In the cryptogenic chronic liver disease cases 36% were positive for anti-C100-3 and 67% were positive for anti-c22 (p < 0.001). Only in one case (a patient with hepatitis B virus infection) was anti-C100-3 detected without concomitant anti-c22. None of the voluntary blood donors had detectable hepatitis C virus antibodies. The new enzyme-linked immunosorbent assay test for anti-c22 would appear to be a more sensitive indicator of chronic hepatitis C virus infection than the existing commercial test, suggesting a useful diagnostic role in both cases of cryptogenic chronic non-A, non-B hepatitis liver disease and for the screening of blood products for prevention of hepatitis after transfusion. (HEPATOLOGY 1992;15:175–179).