One hundred and twenty weanling rats fed diet NIH 2000 that were free of Streptococcus sobrinus and other mutans streptococci were employed in this study. Sixty rats were inoculated orally with S. sobrinus 6715. Each infected rat (donor) was paired and housed with an uninfected recipient. Saliva and serum samples were collected from 24 (12 donor and 12 recipient) rats at the baseline (day 0) and from groups of 12 recipients sacrificed on days 10, 24, 38, and 52, and the level of infection with S. sobrinus was monitored. Salivary immunoglobulin A (IgA) and IgG and serum IgM and IgG antibodies reactive with whole cells (WC), glucosyltransferase (GTF), and the serotype carbohydrate (g) of S. sobrinus were measured by an indirect enzyme-linked immunosorbent assay. Although the rats were free of S. sobrinus and other mutans streptococci at baseline, they exhibited salivary IgA and serum IgM antibodies reactive with S. sobrinus WC, GTF, and g and serum IgG antibodies reactive with WC and GTF. Infection of recipients with S. sobrinus did not induce salivary antibodies reactive with WC, GTF, or g. In contrast, increases in serum IgM and IgG antibodies reactive with WC and serum IgM antibodies reactive with g were observed.