After birth various bacterial species, mainly streptococci, colonize the oral cavity and are frequently isolated from carious lesions in children. Previous studies suggest that an earlier colonization of a child's mouth by cariogenic organisms might be related to a higher risk of caries. The aim of this study is to assess the influence of different milks for infant feeding on the growth of Streptococcus mutans (SM) colonies. Three human milk samples from three different mothers and five infant formulas were tested. To prepare the bacterial inoculum, SM was grown in Brain Heart Infusion broth for 18 hours at 37°C in microaerophilic atmosphere. The growth of SM was determined immediately after the inoculation of milks (T0) and after 24 hours (T24) of incubation. After 24 hours of incubation (ΔT= CFUs/ml at T24 CFUs/ml at T0) the bacterial growth changes were different among milks. Among the complementary milks tested, ΔT of formulas supplemented with Lactobacillus reuteri and with Bifidobacterium lactis was lower than those of non-supplemented formula. In conclusion, on the basis of the reduced SM growth in milks supplemented with probiotics, we may speculate that these formulas have a preventive effect on the development of caries in children.