The final step of keratinocyte differentiation, transition from the granular cells to the cornified cells, involves various post-translational modifications that include deimination of arginine residues. Major deiminated epidermal proteins are derived from K1. Two preferred deimination sites were identified in mouse K1, one in the V1 and the other in the V2 subdomains. An antibody against the deiminated peptide sequence in the V2 subdomain recognized not only deiminated mouse K1 but also deiminated human K1. In this study we analyzed distribution of deiminated K1 in normal human skin and in bullous congenital ichthyosiform erythroderma at light and electron microscopic levels. In normal skin the first few (1-3) cornified cell layers were positive for filaggrin and negative for the antibody against deiminated mouse K1 peptide, whereas the more superficial cells were negative for filaggrin and strongly positive for the antibody against deiminated mouse K1 peptide, indicating slightly delayed onset of K1 deimination at the initial stage of cornification. The clumped keratin in bullous congenital ichthyosiform erythroderma that was not properly compacted with filaggrin was poorly positive to the antibody against deiminated mouse K1 peptide. In addition, K1 derivatives in bullous congenital ichthyosiform erythroderma reacted poorly with the antibody against deiminated mouse K1 peptide compared with the normal control in immunoblot analyses. Our results suggest sequential reorganization of cornified cell keratin filaments involving filaggrin-mediated compaction and K1 deimination. Abnormal keratin aggregation in bullous congenital ichthyosiform erythroderma is likely to disturb the normal deimination of K1.