A key feature of the gingival epithelium is the protection of the periodontal tissue from microbial challenge. Especially in the gingival sulcus area, subgingival extension of a biofilm and the subsequent alterations of the epithelial layer may lead to periodontal destruction and loss of teeth. One of the characteristics of a functional epithelium is the developement of transepithelial electrical resistance (TER), a measurable indicator of epithelial integrity. In order to study the barrier function of epithelium we established an in vitro cell culture system. We used primary gingival keratinocytes and, as a positive control the MDCK I cell line, which is known to form very high TER values. Both cell types were seeded on collagen-coated cell culture inserts, grown to a confluent layer and the development of TER was detected with a volt-ohmmeter. With this system we are able to show an increase of TER which persists for 4-5 days. In order to enhance the TER values and subsequently the barrier function of the model system we used all-trans retinoic acid (RA) which is a potent regulator of epithelial differentiation. The daily application of RA led to an oscillation of the epithelial integrity resulting in an oscillation of the TER values whereas a one fold application of RA resulted in a delayed establishment of TER and the barrier function. Our results indicate that through topical application of RA one of the fundamental properties of the barrier function is profoundly influenced. Keywords: human oral keratinocytes, epithelial barrier, in vitro model