The complete amino acid sequences of hepatitis C virus (HCV) core (residues 1 to 115) and putative matrix (residues 116 to 190) proteins were synthesized as 18-residue-long peptides with an 8-amino-acid overlap. The peptides were assayed with 50 human serum samples with antibodies to HCV (anti-HCV) and 46 serum samples without anti-HCV, as determined by several commercial assays. Immunodominant regions were defined within residues 1 to 18, 11 to 28, 21 to 38, 51 to 68, and 101 to 118. The peptides that covered these regions were recognized by 40 of 50 (80%), 42 of 50 (84%), 36 of 50 (72%), 34 of 48 (68%), and 36 of 48 (72%) of the anti-HCV positive serum samples, respectively. Two anti-HCV negative serum samples were each repeatedly reactive with one peptide, but both were found to be negative by confirmatory anti-HCV assays. Four serum samples that were confirmed to be positive for anti-HCV in commercial assays did not recognize any of the peptides that cover the HCV core-matrix regions. Ninety-two percent of anti-HCV-positive serum samples reacted with a combination of peptides covering residues 1 to 18 and 11 to 28. Testing of peptides that contain the reported genotypic variations of the HCV core within the regions at residues 1 to 18, 51 to 68, and 101 to 118 showed that a change from Thr-110 to Asn-110 decreased the reactivities of eight serum samples. In conclusion, we found that human antibodies to the HCV core-matrix protein(s) are mainly directed to linear determinants and can easily be reproduced by using short synthetic peptides. We also found that such antibodies develop in more than 90% of HCV-infected people.