In addition to its involvement in the formation of the capsid shell of the virus particles, the core protein of hepatitis C virus (HCV) is believed to play an important role in the pathogenesis and/or establishment of persistent infection. We describe here alternative forms of genotype 1b HCV core protein identified after purification of various products of core protein segment 1-169 expressed in Escherichia coli and their analysis by proteolysis, mass spectrometry, and amino acid sequencing. These proteins all result from a +1 frameshift at codon 42 (a different position than that previously reported in genotype 1a) and, for some of them, from a rephasing in the normal open reading frame at the termination codon 144 in the +1 open reading frame. To test the relevance of these recoding events in a eukaryotic translational context, the nucleotide sequences surrounding the two shift sites were cloned in the three reading frames into expression vectors, allowing the production of a C-terminally fused green fluorescent protein, and expressed both in a reticulocyte lysate transcription/translation assay and in culture cells. Both recoding events were confirmed in these expression systems, strengthening the hypothesis that they might occur in HCV-infected cells. Moreover, sera from HCV-positive patients of genotype 1a or 1b were shown to react differently against synthetic peptides encoded in the +1 open reading frame. Together, these results indicate the occurrence of distinct recoding events in genotypes 1a and 1b, pointing out genotype-dependent specific features for F protein.