A fragment of DNA from within the minimum transforming region (mtr-1) of herpes simplex virus type 1 (HSV-1) is known to raise the mutation frequency of cells. This activity has been attributed to a viral protein whose properties are largely unknown. Antiserum was raised to a synthetic peptide of a predicted amino acid sequence from the protein, and was found to react with cells that were infected by HSV-1 in an ELISA and by immunocyto-chemical staining. A combination of immunoprecipitation and immunoblotting techniques confirmed that the epitope is located at the carboxy terminus of the UL26 gene product and is downstream of epitopes that are recognized by two monoclonal antibodies. The mutagenic peptide was different from the conventional gene product of UL26 in that: (a) It was expressed from a different reading frame, (b) It was expressed earlier in infection, and (c) It bound DNA, and thus could be separated by DNA-cellulose chromatography. An RT-PCR experiment revealed two deletions in the cDNA, suggesting that RNA splicing could account for the frameshift. Examination of the DNA sequence of the region also revealed a potential ribosomal frame-shift site. The mutagenic peptide of HSV-1 is therefore a product of the UL26 gene which is expressed with a different carboxy terminus early in infection, and this could be due either to RNA splicing or to ribosomal frame-shifting.