Porphyromonas gingivalis, a gram-negative anaerobe, is one of the major causative agents of periodontal disease. In this study, the effects of chlorhexidine digluconate and hydrogen peroxide on the hemin binding of P. gingivalis and coaggregation of this bacterium with oral streptococci were examined. The pretreatment of P. gingivalis W50 and 381 with chlorhexidine digluconate and hydrogen peroxide increased the hemin binding of these bacteria. The hemin binding of P. gingivalis was increased by the subminimal inhibitory concentration (MIC) of chlorhexidine digluconate. However, concentrations of hydrogen peroxide below the MIC had no effect on the hemin binding of P. gingivalis W50 and 381. Coaggregation of P. gingivalis 381 with Streptococcus oralis ATCC 9811 and Streptococcus gordonii DL1 was diminished by chlorhexidine digluconate. The coaggregation-inhibitory effect was concentration-dependent. Hydrogen peroxide also showed inhibitory effects on the coaggregation of P. gingivalis 381 with S. oralis 9811 and S. gordonii DL1 at concentrations below that used clinically. Concentrations of chlorhexidine digluconate below the MIC inhibited coaggregation. However, concentrations of hydrogen peroxide below the MIC were not effective in reducing the coaggregation of P. gingivalis with oral streptococci. These observations show that chlorhexidine digluconate and hydrogen peroxide could confer variable effects on P. gingivalis hemin binding and coaggregation of this bacterium with oral streptococci.