SL-diet was prepared using commercial pellets supplemented with 1% pulverized Streptococcus lactis (SL). SL fed group of ICR strain mice were bred from 8 weeks up to 75 weeks. End-point age for survival in these mice was approximately 75 weeks. No significant differences in survival rate were observed at 75 weeks between the commercial diet and SL-diet group. The superoxide dismutase (SOD) activities in the liver, brain and erythrocytes of 75-week-old mice significantly decreased compared with those aged 10 weeks. Cu.Zn-SOD and Mn-SOD activities in the liver and brain of SL-diet mice were maintained in similar degree in the juvenile group, however, decreased significantly in the commercial diet group. SOD in erythrocytes was slightly decreased and no significant difference was noted in either group.