Second-generation hepatitis C virus (HCV) ELISAs are currently in use in Europe and have been submitted for approval in the United States. These new assays contain additional antigens from the putative nucleocapsid and NS-3 regions of the HCV genome in addition to the c100-3 antigen present in first-generation ELISAs. A supplementary test, the second-generation RIBA HCV strip immunoblot assay (2-RIBA HCV SIA) has also been developed. The strip immunoblot assay uses four recombinant HCV antigens [5-1-1 (NS-4), c100-3 (NS-4), c33c (NS-3), and c22-3 (NS-3) (nucleocapsid)] slot blotted on nitrocellulose. Screening of random volunteer blood donors with the Ortho second-generation HCV ELISA (ORTHO HCV 2.0 ELISA) indicates that a substantial change in the repeat reactive donor population is observed with the new test. Two notable features of this change are: (1) A large number of samples reactive in the 2-RIBA HCV SIA for the second-generation antigens, c33c and c22-3, are detected by the ORTHO HCV 2.0 ELISA; (2) the percentage of ORTHO HCV 2.0 ELISA reactive specimens found indeterminate (reactive for only one HCV antigen) by the 2-RIBA HCV SIA is higher than in first-generation HCV ELISAs (approximately 25 vs. 5%). In addition, ORTHO HCV 2.0 ELISA repeat reactive, 2-RIBA HCV SIA indeterminate samples are dominated by reactivity to c22-3 instead of c100-3, which is the case for first-generation HCV ELISA repeat reactive samples. Resolution of 2-RIBA HCV SIA indeterminate samples as either containing anti-HCV antibodies or not, is important in both diagnostic and blood screening environments, especially where donor notification is required. Our approach to resolution of these troublesome samples evolved from initial work with HCV peptides. Early studies with an experimental strip immunoblot assay containing 5 peptides from the nucleocapsid, E2 (NS-1), NS-4, and NS-5 regions of the viral genome indicated that peptides from the nucleocapsid and NS-4 regions of the genome could provide additional evidence for the presence of anti-HCV antibodies with good specificity, but other peptides suffered from poor specificity. In addition, no immunoreactive peptide from the NS-3 (c33c) region of the virus is available, presumably because the major epitope(s) of this key second-generation antigen is a conformational determinant.(ABSTRACT TRUNCATED AT 400 WORDS)