Methylene blue, at a concentration of 1.0 mg/ml, was able to raise the redox potential (E(h)) of pre-reduced culture medium from - 288 mV to -154 mV and to poise the medium at an E(h) of - 171 mV during 48-h incubation in an anaerobic atmosphere. Addition of the dye to suspensions of the anaerobic periodontopathogen, Porphyromonas gingivalis, to give a final concentration of 1.0 mg/ml raised their E(h) to - 125 mV. During a 24-h period of anaerobic incubation, the dye maintained the E(h) at a level (- 150 mV) that was more than 200 mV higher than control, dye-free suspensions, and this was accompanied by at least a 6-log reduction in the viable count. The bactericidal effect of the methylene blue could be counteracted by the reducing agent dithiothreitol at a concentration of 2.0 mg/ml, which lowered the E(h) to a level (- 333 mV) similar to that of controls. These results have demonstrated that the presence of 1.0 mg/ml methylene blue, in its oxidized form, in an environment is inimical to the survival of P. gingivalis and that this is related, directly or indirectly, to its ability to raise the E(h) of the environment. Topical application of methylene blue, or other redox agents, may be an effective means of eliminating this organism from the periodontal pockets of patients with chronic periodontitis.