OBJECTIVE: To investigate the effect of local application of monoclonal antibody (McAb) raised against the 210KD cell surface protein antigen (PAg) of Streptococcus sobrinus on the colonization of rats' teeth and the development of dental caries. MATERIALS AND METHODS: Thirty Wistar rats were divided into 3 groups: group 1 received McAb against PAg; group 2 received unrelated mice ascites; group 3 received pure buffered saline. The bacteria were applied 6 times to the occlusal surfaces of molars and to the labial surfaces of incisors. After three applications, streptomycin-resistant S. sobrinus 6715 was applied to the teeth and the rats were fed with caries-inducing diet 2000. The number of colonized S. sobrinus was counted on the streptomycin-supplemented mutans streptocci agar and expressed as a percentage of the total colonies on BHI agar. The presence of caries was examined and scored following Keyes' procedure. RESULTS: There were significantly lower levels of colonization by S. sobrinus on rats' teeth and lower mean caries scores on the teeth treated with McAb compared with those in the control groups. CONCLUSION: Local passive immunization with McAb against PAg may be an effective way to prevent colonization by implanted S. sobrinus and development of dental caries.