The safety of two probiotic bifidobacterial strains, Bifidobacterium breve M-16V and Bifidobacterium infantis M-63, was evaluated by single dose and 90-day repeated dose oral toxicity tests using rats. In the single dose oral toxicity test using 1.4 × 10¹² CFU/kg of B. breve M-16V or 3.2 × 10¹¹ CFU/kg of B. infantis M-63, there were no death and no abnormalities. In the 90-day repeated dose oral toxicity test using 2.3 × 10¹¹ CFU/kg/day of B. breve M-16V or 7.6 × 10¹⁰ CFU/kg/day of B. infantis M-63, no death and no abnormalities in body weight, food consumption, water consumption, urinalysis, hematology, blood biochemistry, organ weights, and histophathological findings were observed. The acceptable daily intake (ADI) of B. breve M-16V was calculated to be 1.38 × 10¹¹ CFU/day for an adult weighing 60 kg and 1.15 × 10¹⁰ CFU/day for an infant weighing 5,000 g. Based on the findings of the present study together with the taxonomy and distribution data, as well as clinical experience in preterm infants, B. breve M-16V may be considered as extremely safe for consumption by humans including infants. Although B. infantis M-63 was considered as safe as B. breve M-16V, further confirmation by clinical investigations may be required.