Twenty-eight children, aged 0.8-3.8 years, were studied with respect to the presence of mutans streptococci (MS) in dental plaque and the amount and avidity of specific serum IgG and IgM antibodies against whole cells and streptococcal protein antigen I/II (SA I/II) of Streptococcus mutans. The presence or absence of MS in dental plaque at the median age of 1.9 years predicted the caries development during a 2.7-year follow-up period with a sensitivity and specificity of 73 and 92%, respectively. The caries predictive value of a positive finding of MS at this age was as high as 92%. Almost all children had detectable amounts of serum IgG and IgM antibodies against whole cells and SA I/II of S. mutans, irrespective of the presence of detectable levels of this bacterium in dental plaque. These antibodies increased with age. The antibody levels did not differ significantly between children who were MS-negative or MS-positive. However, in MS-free children as well as in the whole study group, total specific and high-avidity antibodies of IgG class against whole cells correlated positively with antibodies against SA I/II. In MS-infected children such an association was observed for IgM but not for IgG antibodies. This different serum antibody profile in MS-negative and -positive children may be related to the mode of immunization with S. mutans.