Formula-fed infants are more susceptible to infectious diseases because they lack the maternal immune factors transferred from breast milk, while their own immune system is still immature. As timely probiotic administration was suggested to promote immune system development in formula-fed infants, this study aimed at assessing the safety and the effects of a probiotic supplement (Bifidobacterium infantis R0033, Bifidobacterium bifidum R0071, and Lactobacillus helveticus R0052) on mucosal immune competence and digestive function in formula-fed infants. Healthy infants (3.5-6 months old) were randomised to receive either probiotic- (n=66) or placebo-supplemented (n=66) formula once a day for four weeks. In the probiotics group, faecal secretory immunoglobulin A (SIgA) levels remained similar between visit 2 (baseline; V2) and visit 3 (end-of-treatment; V3), but decreased in the placebo group. Changes in SIgA levels following treatment (log(10)Delta V3-V2 [95%CI]) between the probiotic and placebo groups were statistically significant (23 ng/dl [-57;102] and -137 ng/dl [-212;-62], respectively (P=0.0044; ANCOVA)). While log(10)Delta V3-V2 [95%CI] for salivary SIgA levels increased in both groups, this trend was more pronounced in the probiotics than in the placebo group with an increase of 123 ng/dl [9;236] and 37 ng/dL [-72;147], respectively (P=0.2829; ANCOVA). The weekly average number of stools/day was significantly higher in the probiotics group compared to placebo during the last week of treatment for the per protocol population. There was no difference in microbiota composition or anthropometric parameters between groups. No serious adverse event was reported, and all adverse events were mild and unrelated to the product or study. Our results show that formula-fed infants receiving probiotics maintained higher faecal SIgA levels at the end of the four-week treatment period, suggesting a positive effect of probiotics on SIgA production. This study demonstrates the safety of this probiotic formulation in infants. Formula-fed infants may benefit from probiotics supplementation to sustain the development of mucosal immunity.