The cytomorphological effects of folic acid were studied using in vitro established human oral epithelium. It was demonstrated that a concentration twice that used clinically (200 micrograms/ml) did not induce marked cytotoxic reaction in the cultured cells. The most pronounced changes were observed in cultures exposed to 200 micrograms/ml folic acid both in primary culture and subculture. The cultures displayed areas of degenerating cells showing oedema and increased translucency of the cytoplasm, flattened cells with distinct tonofilaments and atypical mitotic figures. Identical changes were found in cultures exposed to 50 and 100 micrograms/ml folic acid but to a lesser extent than in 200 micrograms/ml. These changes indicated that folic acid increased the number of cells undergoing terminal differentiation. From this study we suggested that folic acid when applied topically may play a role in local stimulation of epithelial cell differentiation leading to enhanced healing of oral ulcers.