Porphyromonas gingivalis, an obligate anerobe with a growth requirement for iron protoporphyrin IX (FePPIX), is exposed to increased temperatures in the inflamed periodontal pocket. In this study, P. gingivalis was grown in a chemostat at 37 degreesC (control), 39 degreesC, and 41 degreesC, and examined for hemagglutinating (HA) activity, hemoglobin binding and degrading activity, and iron protoporphyrin IX binding. HA activity decreased in cells as the growth temperature increased. Binding of mu -oxo bishaem (dimeric haem), and Fe(II)- and Fe(III)-monomeric forms was increased in 39 degreesC-grown cells but decreased in 41 degreesC-grown cells compared with controls. Cellular hemoglobin binding and degradation decreased with increased growth temperature. The decrease in cellular hemagglutination and hemoglobin degradation occurring with increased growth temperature would limit the potential overproduction of toxic monomeric haem molecules. The increased binding of mu -oxo bishaem and monomeric forms of FePPIX at 39 degreesC may reflect a defense strategy against reactive oxidants and a mechanism of dampening down the inflammatory response to maintain an ecological balance.