The preventive effects of retinoids on oral carcinogenesis may be related to their ability to modulate the growth and differentiation of human oral squamous epithelial cells. Nuclear retinoid receptors (RAR α, β, and γ, and RXR α, β, and γ) may mediate these effects by regulating gene transcription. The removal of serum from the growth medium of two head and neck squamous cell carcinoma lines 1483 and SqCC/Y1 resulted in a decrease in RARβ mRNA level and concurrent increases in the expression of the keratin K1 and transglutaminase type I (TGase I), which are markers of differentiation of keratinizing squamous epithelial cells. All-trans-retinoic acid (tRA) or 13-cis-RA increased RARβ and decreased K1 and TGase I mRNA levels in serum-free medium. Transcriptional activation of reporter genes by means of retinoid response elements (RARE and RXRE) indicated that the RXR-RAR pathway predominates over the RXR homodimer pathway in the 1483 cells. Among several synthetic retinoids with preference for binding to specific nuclear retinoid receptors, those that induced RARβ also suppressed K1. The inverse association between RARβ expression and K1 and TGase I levels implicates this receptor in suppression of keratinization in oral epithelial cells.