The aim of this study was to determine the degree to which the epidermis and oral epithelium of species other than man express cytokeratin (CK) intermediate filaments, which are markers of epithelial differentiation. Fixed, wax-embedded samples of skin, buccal mucosa and gingiva from rhesus monkey, marmoset, cow, sheep, pig, ferret, hamster, axolotl and trout were tested for CK expression using a panel of antihuman CK antibodies and an immunoperoxidase procedure. Human skin and oral mucosa were also stained to act as positive control. The results showed that antihuman CK antibodies stained animal tissues, but the patterns of staining were not always identical to the established human CK profile. Of particular interest was the expression of CK18, typically only detected in ‘simple’ epithelium in man, in bovine, ferret and hamster stratified epithelium from different sites. However, there was evidence of variable anti-CK antibody cross-reactivity, both as a result of intrinsic variations in CK polypeptide structure and as artifacts of fixation. We conclude that some CK are conserved between species, but that biological variables, for example local functional requirements, and technical factors affect the results. These considerations need to be borne in mind in animal studies of epithelial differentiation employing CK immunohistochemistry. Biochemical characterisation is ultimately necessary to determine specific differences between human and animal CK.