We have performed a modified Boyden chamber chemotaxis assay using NRK-49F cells under serum-free condition, in order to characterize the chemotactic activity of recombinant platelet-derived growth factor (PDGF)-AA, -AB, and -BB. All three PDGF isoforms showed similar chemotactic activity on NRK cells expressing both PDGF alpha- and beta-receptors. A checkerboard analysis revealed that PDGF-AA was a true chemoattractant for NRK cells, indicating that alpha-receptors could transduce signals for the chemotactic response. We then examined the inhibitory effects of PDGF isoforms in the upper chamber on NRK cell migration to PDGF in the lower chamber. When higher concentrations of PDGF-AB or -BB were present in the upper chamber, cell migration to any of the PDGF isoforms was completely blocked, whereas PDGF-AA in the upper chamber could not induce complete inhibition of the chemotactic migration to PDGF-AB or -BB. Even when 100 ng/ml of PDGF-AA was present in the upper chamber, NRK cell migration to 5 ng/ml of PDGF-AB in the lower chamber was not completely blocked. These findings suggest that the chemotactic response induced by ligand binding to PDGF beta-receptor may be different from that induced by ligand binding to PDGF alpha-receptors.