We examined the effect of the concentration of various types of iron molecules on the regulation of growth of Porphyromonas gingivalis. Bacterial growth was monitored spectrophotometrically. The hemin-depleted cells of P. gingivalis 381 were incubated in the basal medium plus test substrates such as hemoglobin, hemin, transferrin and various inorganic iron compounds. The relationship between the specific growth rate of organisms and the concentration of iron-containing compounds was determined. The value of Ks, a parameter analogous to the Michaelis-Menten constant, was estimated. P. gingivalis 381 showed a Ks value of 3.85, 4.91 and 0.0017 microM for hemin, transferrin and hemoglobin, respectively. However, the inorganic iron compounds tested did not support growth of P. gingivalis. These findings suggest that P. gingivalis utilizes hemoglobin as an iron source much more effectively than other iron-containing compounds under an iron-limited environment.