Epidermolytic hyperkeratosis is an autosomal dominant disorder affecting the structural integrity of the suprabasal layers of human epidermis. We have recently documented in one family linkage of the disease phenotype to the cluster of type II keratins. We have now identified a leucine-prollne amino acid substitution in the conserved Hl subdomain of keratin 1 that is present only in affected family members. Using a quantitative assay and electron microscopy with synthetic peptides, we show that, whereas the wild-type Hl peptide rapidly disassembles preformed keratin filaments in vitro, the mutant peptide does this far less efficiently. Therefore the mutation in keratin 1 is likely to cause defective keratin filaments and hence a defective cytoskeleton in the epidermal cells in vivo.