Due to potential side-effects of active immunization by cariogenic mutans streptococci, oral administration of passively-derived antibodies could be a more acceptable way to reduce colonization and virulence of these microorganisms in human dentition. The aim of this study was to produce antistreptococcal immunoglobulins into bovine colostrum and explore the possible antibacterial mechanisms of these immunoglobulins against mutans streptococci. Specific serum IgG antibodies to whole cell antigens of both Streptococcus mutans and Streptococcus sobrinus increased rapidly in cows during immunization and were high also in the final whey-product. Low concentration (0.5% w/v) of bovine immune preparation inhibited significantly the incorporation of [14C]glucose by both S. mutans and S. sobrinus. Higher concentration (> 1%) was needed to inhibit the glucosyltransferase or fructosyltransferase activities of these bacteria. No such inhibitory effects were observed with the control preparation from the non-immunized cows. Our results indicate that bovine immune colostrum has a significant inhibitory potential against mutans streptococci, apparently dependent on the presence of specific IgG antibodies against S. mutans and S. sobrinus.