Citrulline residues are detected in keratins and filaggrin in the cornified layers of mammalian epidermis. Such citrulline residues are formed by the enzymatic deimination of arginine residues by peptidylarginine deiminase (EC 126.96.36.199). Major deiminated keratins are thought to be partially degraded/disulfide-cross-linked keratin K1 based on the immunoblotting profiles. In order to obtain more definitive evidence of the deimination of keratin K1 and also to investigate its functional significance, we attempted to identify its preferred acting sites of peptidylarginine deiminase. A partially degraded keratin K1 fraction obtained from the cornified layer of newborn mouse epidermis was subjected to limited proteolytic cleavages, and the resulting deiminated peptides were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or reverse-phase high-performance liquid chromatography for N-terminal sequencing and/or amino acid analysis. At least two sites were identified, one in the V1 and the other in the V2 subdomains of keratin K1. An undecapeptide sequence covering the latter shows about 70% homology with an undecapeptide sequence in the V2 subdomain of human K1, a presumptive site of deimination. We speculated that the deimination of arginine residues in these subdomains might modulate their interactions with epidermal proteins other than keratins and filaggrin during the terminal stage of epidermal differentiation.