Thirty-one children, aged 0.8–3.8 years, were studied with respect to dental caries, the presence of Streptococcus mutans in dental plaque, and the amount and avidity of specific antibodies against S. mutans serotype c in serum and saliva. Twelve of the children harbored S. mutans in plaque and all 4 children with carious lesions belonged to this group. Comparison of S. mutans-positive and -negative children revealed no significant differences in the concentrations of total immunoglobulins (isotypes A, G and M) in serum or saliva, nor did the amount of intact secretory IgA molecules in saliva differ between the groups. All children had developed serum antibodies against S. mutans, irrespective of the presence of this bacterium in their plaque samples. Significantly more specific serum antibodies of IgG type (p = 0.001) were detected in S. mutans-free children than in those whose dentition was infected with this bacterium. Specific salivary IgA antibodies to S. mutans were found in only 26% of all children (8/31) with no correlation to the presence of oral S. mutans. Our results could be explained by the observation that the oral exposure to antigens in early infancy results preferably in a systemic (serum) immune response, which may be due to the incomplete antigen exclusion in the intestinal epithelium of human neonatal infants. Furthermore our data indicate that the specific serum IgG antibodies may be protective against the colonization of primary teeth by S. mutans in early childhoo