The growth factors PDGF-AA and PDGF-BB have previously been shown to be potent mitogens for human periodontal ligament (hPDL) cells in vitro. Additionally, the mitogenic response to PDGF-AA has been shown to be specifically inhibited by TGF-P. The purpose of the present investigation was to examine the binding of PDGF-AA and PDGF-BB, and the modulation of PDGF binding by TGF-P, in hPDL cells. Scatchard analysis identified an average of 32,000 PDGF-AA high-affinity binding sites per cell with a dissociation constant (Kd) of 0.66 nM and an average of 36,000 PDGF-BB binding sites per cell with a dissociation constant (k,) of 0.44 nM. After treatment with TGF-P, the receptor number for PDGF-AA was found to specifically decrease by approximately 50%, with no change in binding affinity. This reduced number of binding sites was shown to correlate with both a decrease in levels of receptor tyrosine phosphorylation and a decreased number of a receptor subunits. Northern blot analysis identified the TGF-P-mediated decrease in PDGF a receptor subunit mRNA levels. PDGF-BB showed little change in the number of binding sites or in the binding affinity with TGF-P treatment, and the data were consistent with an increase in the number of P receptor subunits. These results demonstrate nearly equivalent numbers of receptors for both PDGF-AA and PDGF-BB in hPDL cells. Also, modulation of PDGF binding, by TFG-p, was shown to result in a reduced number of a receptor subunits with an increase in the number of (3 receptor subunits.