Expression of retinoic acid receptor β (RARβ) is reported to be absent or down-regulated in oral squamous cell carcinomas. Recently, we found that the growth-inhibitory effect of 9-cis-retinoic acid (9CRA) on oral squamous cell carcinoma may depend on the expression levels of endogenous RARβ. In order to clarify the role of RARβ in growth and differentiation, we transfected RARβ expression vector into oral squamous carcinoma cell lines, HSC-4 and Ho-1-N-1. Both RARβtransfected cell lines displayed growth inhibition. Moreover, RARβ-transfected clones underwent morphological changes, and RARβ-transfected HSC-4 clones underwent apoptosis even in the absence of 9CRA treatment. In contrast, RARβ-transfected Ho-1-N-1 clones exhibited cell cycle arrest without undergoing apoptosis initially; however, apoptosis was induced in these cells after 6 days of 9CRA treatment. RARα and RARγ expression was reduced at both the protein and mRNA levels in RARβ transfectants, whereas the expression of retinoid X receptor α (RXRα) was not altered. RARβ transfectants exhibited alterations in the levels of cell cycle-associated proteins, histone acetyltransferase (HAT) and apoptosis-associated proteins. After 6 days of 9CRA treatment, RARβ transfectants overexpressed Waf1/Cip1/Sdi1/p21, Kip1/p27, chk1, p300/CBP, BAX, Bak, Apaf 1, caspase 3 and caspase 9. Conversely, E2F1, cdc25B and HDAC1 were down-regulated in these transfectants. In addition, histone H4 acetylation was induced in RARβ transfectants. These findings suggest that histone acetylation mediated by histone acetyltransferase and p300/CBP may play a role in the growth arrest and apoptosis induced by RARβ transfection in oral squamous cell carcinoma.