Keratins constitute a complex and diverse family of proteins that are expressed in a tissue-specific and differentiation-specific fashion. In some cells, retinoids can influence both keratin expression and cell differentiation. In this study of the molecular mechanisms underlying the differential expression of keratins, we examined keratin proteins, mRNAs, and nascent transcripts in a variety of cultured human cell types, including epidermal cells and simple epithelial cells. We also investigated intermediate filament gene expression in fibroblasts, which express vimentin rather than keratin. We found that both cell type-specific and retinoid-mediated expression of keratins are regulated at least in part at the transcriptional level in these cells. However, given the kinetics of the retinoid-mediated response, it seems most likely that the keratin gene expression that is regulated by retinoids reflects a secondary rather than primary response.