Oxygen (O2) uptake and release of O2 metabolites to the extracellular medium were studied with representatives of serotypes a through g of Streptococcus mutans. When incubated with glucose, washed cells of all strains took up O2 at rates proportional to the O2 concentration. When O2 was held constant at 0.2 mM, 0.2 to 0.5 mol of O2 was taken up per mol of glucose metabolized. Despite the similar rates of O2 uptake, the strains fell into three classes according to the amount of hydrogen peroxide (H2O2) released. Strains BHT, FA-1, and OMZ-176 released up to 90% of O2 taken up as H2O2, which accumulated in the medium to concentrations as high as 2 mM. The high levels of H2O2 accumulation were correlated with low ability to reduce exogenous H2O2 to water. Strains Ingbritt and B-13 released about half as much H2O2, but H2O2 in the medium did not exceed 0.05 to 0.1 mM. Strains HS-6, AHT, GS-5, OMZ-175, LM-7, and 6515-15 released less than 10% of O2 taken up as H2O2, and H2O2 did not accumulate. Within this class, strains HS-6 and AHT released about 6% of O2 taken up as superoxide (O2-). Release of O2 metabolites was correlated with enzyme activities in cell-free extracts. Extracts from all strains catalyzed NADH-dependent O2 uptake. Extracts from H2O2-accumulating strains produced H2O2 when incubated with NADH and O2 and had low ability to catalyze NADH-dependent reduction of H2O2. Extracts from HS-6 and AHT had low superoxide dismutase activity, which may account for O2- release and the O2-sensitive growth of these strains.