Lipopolysaccharides (LPS) prepared from periodontopathic bacteria have been known to induce various biological responses which may lead to periodontal tissue breakdown. The purpose of this study was to determine if Porphyromonas gingivalis LPS could affect cellular functions of human periodontal ligament fibroblasts (HPLF). We showed here the responsiveness of cultured HPLF to platelet-derived growth factor (PDGF)-BB, a growth factor for mesenchymal cells, in the presence of P. gingivalis LPS. DNA synthesis of HPLF was enhanced in a dose-dependent manner when LPS were co-incubated for 48 h; thereafter, it decreased to the baseline level within 24 h incubation. The stimulating effect of PDGF-BB was further enhanced by the pretreatment of HPLF with LPS (10 μg/ml) for 48 h. The binding assay of [125I] PDGF-BB and the flow cytometric assay using rabbit antiserum to human PDGF receptor (PDGF-R) β-type indicated that this enhancement was due to the increase of the number of PDGF-R β-type on HPLF. Immunoprecipitation using antiserum to human PDGF-R β-type also showed that the synthesis of PDGF-R β-type was augmented in the LPS-treated HPLF. These results indicate that P. gingivalis LPS stimulate cellular proliferation and responsiveness to PDGF-BB of cultured HPLF. These cellular reactions may be mediated by PDGF-BB binding, followed by increased synthesis of the receptor protein.