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. Palabras clave: human oral keratinocytes, epithelial barrier, in vitro model