In this study, we characterized the binding of transferrin to Porphyromonas gingivalis using a classical receptor-binding assay, and examined the relationship between the binding and availability of transferrin for the growth of P. gingivalis. The binding of 125I-labeled human transferrin to P. gingivalis occurred rapidly, reversibly and specifically. Scatchard analysis yielded a Kd of 1.37 ± 0.16 μM and an apparent number of 1.13 ± 0.26 × 105 receptors per cell. The binding of transferrin was much increased when organisms were grown in iron-limited conditions. Among the species of black-pigmented anaerobic rods, those strains of P. gingivalis which had high transferrin-binding activity exhibited unrestricted growth following the addition of transferrin to the hemin-free culture medium. On the other hand, the presence of transferrin in the culture medium did not support unrestricted growth of organisms that had low transferrin-binding activity. These results suggest that the binding of transferrin to P. gingivalis cells may be a preliminary step in iron acquisition, which allows them to survive in the healthy periodontal environment.