The signals for epidermal differentiation and barrier formation are largely unknown. One possible signal is dehydration or osmotic stress. To test this hypothesis, we investigated the effects of osmotic stress on markers of differentiation of normal human keratinocytes in culture. Hyperosmotic stress treatment of normal human keratino-cyte cultures by elevated sorbitol concentrations was observed to induce markers of terminal differentiation. Sorbitol was added to keratinocyte media at 50, 100, 200, and 300 mM final concentration. These concentrations of sorbitol induce a dehydration effect or osmotic stress on the keratinocytes. These sorbitol treatments increased the levels of messenger RNA for the differentiation markers involucrin, transglutaminase, and filaggrin as measured by reverse transcription-polymerase chain reaction. Keratin K1 and K10 and involucrin protein levels were also increased in normal human keratinocyte cultures exposed to increasing osmotic stress. These observations suggest that keratino-cytes in the epidermis may use dehydration as a sign to trigger the differentiation of the skin barrier.