Triclosan is known to reduce the untoward side effects of sodium lauryl sulphate (SLS). The aim of the present study was to determine whether triclosan can inhibit SLS-induced changes in expression of cytokeratin (CK) genes in hamster cheek pouch epithelium. With a hybridohistochemical technique, using specific human cRNA probes, hamster CK mRNAs were identified by immunological detection of heterologous hybrids. In contrast to application of SLS-containing paste, application of paste containing SLS together with triclosan produced no marked changes in expression and distribution patterns of CK mRNAs, compared to the normal cheek pouch epithelium. Therefore, we may accept that triclosan inhibits the effect of SLS on CK gene expression. However, the mechanism of this protection remains elusive. Conversely, the epithelial hyperplasia induced by application of SLS was histologically identical to that induced by application of SLS and triclosan. This suggests that the changes in CK gene expression identified in the present study are not a simple consequence of epithelial hyperplasia, but rather are specific to the irritating agent. Establishment of the fact that SLS may influence gene expression, and that this may be prevented by triclosan, may be helpful in research on the cytological effects of SLS and the elucidation of protection mechanisms of triclosan against side effects of SLS.