An expression library established in lambda gt11 with cDNA from squamous epithelium of the human upper digestive tract was screened with an antibody raised against keratin 13. A 1.2 kb fragment from the most strongly reacting plaque was sequenced and compared to known type I keratin sequences. The highest degree of homology was detected with the murine 47K type I keratin, which we consider to be the counterpart of human keratin 13. Tryptic peptides of keratin 13 were separated on a HPLC column and one peptide was sequenced. The amino acid sequence obtained supports the identity of the cDNA. An eight codon motif has been tandemly repeated in the C-domain of keratin 13. In spite of substantial divergence by point mutations and deletions, the remaining sequence homologies suggest that the C-domains of both the human keratin 13 and the orthologous murine protein have originated from a common ancestor.