Many of micro-organisms under aerobic conditions showed oxygen tolerance caused by the induction of several oxygen-derived free radical scavengers. In the present study, we tried to clarify the changes of active oxygen metabolism of streptococcus mutans (sobrinus) B13 under aerobic conditions. Aerobic and anaerobic culture of streptococcus mutans (sobrinus) B13 was made in normal, metal chleted, and metal supplemented medium. Superoxide dismutase (SOD) activity and bacterial proliferation were determined on the cultured cells. We obtained the following results. 1) In the anaerobic normal medium, SOD activity was not detected, but proliferation was observed. On the other hand, the neither were detected in the metal chleted medium. 2) No effect was observed on the induction of SOD activity and proliferation by the supplementation of copper in the metal chelated medium. 3) We observed positive effects on the induction of SOD activity and proliferation by the supplementation of iron, managnese, or both in the metal chlated medium under aerobic condition. Based on the above results, we found that iron and manganese were essential metals for SOD activity induction. Moreover, we suggested that there was close association of the aerobic bacterial proliferation with the induction of oxygen-derived free radical scavengers.