Keratinocyte growth factor (KGF) is a fibroblast growth factor which mediates epithelial growth and differentiation. KGF is expressed in subepithelial fibroblasts, but generally not in fibroblasts of deep connective tissue, such as fascia and ligaments. Here we demonstrate that KGF mRNA is expressed in periodontal ligament fibroblasts, and that the expression is increased upon serum stimulation. Fibroblasts from human periodontal ligament, from buccal mucosa, from gingiva, and from skin were established from explants. Alkaline phosphatase activity was used as an indicator of the periodontal nature of fibroblasts. Cells were first cultured in DMEM with 0.5% fetal calf serum (FCS) and then incubated for 8 h, and 72 h in fresh DMEM with 10% FCS. Total RNA was isolated and used for Northern blotting with a P32-labeled KGF cDNA probe. Total RNA from cultured keratinocytes was used as negative controls. KGF mRNA was found in all cultured fibroblasts. Upon addition of 10% FCS to the cell cultures, an increase in KGF mRNA levels was noticed especially after 72 h. Furthermore, RT-PCR analysis of material scraped from the tooth root surface indicated the presence of KGF mRNA even in noncultured periodontal ligament cells.