We examined sequential serum samples from 12 patients with well-characterized posttransfusion non-A, non-B hepatitis who had an acute, resolving self-limited type of clinical course for the presence of antibody to the hepatitis C virus nucleocapsid (core) protein (p22) expressed by a recombinant baculovirus. These sera were simultaneously examined for antibody to the hepatitis C virus nonstructural protein (C 100-3) that is presently used for blood screening worldwide. In three patients, both anti-p22 and anti-C 100-3 antibodies were detected, but anti-p22 was detected much earlier. In four patients, only anti-p22 was detected. Two other patients were considered to be hepatitis C virus carriers who had been already infected with hepatitis C virus. In one patient, only anti-C 100-3 was detected, and it was transient. In two patients, neither antibody was detected. Anti-p22 was detected in at least one of eight samples of transfused blood. Of the nine samples of donated blood that were positive for anti-p22, only four were positive for anti-C 100-3. This new assay detecting the antibody to the p22 protein is thus useful for the serodiagnosis of non-A, non-B hepatitis in the acute phase and for blood screening. (Hepatology 1992;15:391–394).