The gastrointestinal (GI) tract of a fetus in utero is sterile but it becomes colonized with environmental microorganisms shortly after birth. Since the gut microbiota undergoes substantial changes in early life, healthy gut microflora is essential to an infant's gut health and immune system and probably also has an effect on overall health status in later life. Probiotics, defined as viable microbial preparations that have a beneficial effect on the health of the host, represent a rapidly expanding field. Although randomized controlled trials using probiotics in infants have shown promising results in the prevention and treatment of common diseases such as diarrhea and allergy, little is known about whether probiotics could offer benefits to healthy infants. We have designed a randomized controlled trial to test the hypothesis that an oral preparation of probiotics is superior to placebo in improving digestive and immune function in healthy infants.The trial will be a randomized, double-blind, placebo-controlled, 2-parallel-group study in Shanghai, China. After a 2-week run-in period, 200 exclusively formula-fed healthy infants aged 4 to 6 months will be randomly allocated to receive either a probiotic product containing Bifidobacterium infantis R0033, Bifidobacterium bifidum R0071, and Lactobacillus helveticus R0052 or an identical placebo once daily for 4 weeks and will be followed up for 8 weeks. The duration of the subject's participation will be 14 weeks, with a total of 5 visits: inclusion (Visit 1, Day 1), start of intervention (V2, D15), end of intervention (V3, D44), and follow-up (V4 and V5, D72 and D100). Stool and saliva samples will be collected at the first 3 visits to measure microbial populations and secretory immunoglobulin A (SIgA), respectively. Physical examination will be performed at each visit, and tolerance records will be completed 1 day prior to each visit. The primary endpoints will be the changes in the composition of fecal microbiota, particularly the Bifidobacterium bifidum population. The secondary endpoints will include the change in salivary SIgA level, growth parameters, digestive tolerance, and adverse events.An effective, practical, and acceptable probiotic intervention in manipulating the gut microbiota and boosting the immune system in formula-fed infants would represent a major clinical advance. The administration of probiotic supplementation or follow-on formula to infant may be associated with some clinic benefits.