 The proposed phase I clinical trial is a parallel-group, placebo-controlled, randomized, double-blind ascending dose study of dietary supplementation with Bifidobacterium longum subsp. infantis B. infantis in healthy breastfed infants to evaluate its safety as well as determine the pharmacologically effective dose ED of B. infantis producing at least 50 gut colonization at six weeks of age. Infants will be enrolled sequentially in groups of five three randomized to receive B. infantis and two to receive placebo. For each group, infants will be dosed with B. infantis or placebo on day 7 and day 14 of life. A calculated Maximally Recommended Starting Dose MRSD will be used to initiate the dose escalation and is defined below. Every two weeks, an additional group of five infants randomized 3:2 to B. infantis and placebo will be enrolled to receive progressively higher doses of B. infantis. Calculation of the appropriate dose escalation will be performed using a modified Fibonacci series as described below in an effort to identify the ED of B. infantis. 