Sprague-Dawley rats fed diet 2000, infected by Streptococcus sobrinus, and offered milk or lactose-reduced milk as drinking fluids developed essentially similar caries scores as controls given distilled water. In contrast, animals given fructose or sucrose solutions developed enhanced levels of caries. Significant differences were not detected in the S. sobrinus populations among the groups. In further experiments, the addition of sucrose to milk enhanced the severity of caries scores. However, sucrose-milk solutions were significantly less cariogenic than sucrose-water solutions. Animals fed sucrose (2-5%) milk solutions harbored enhanced populations of S. sobrinus. The results, overall, suggest that milk may have modest cariostatic properties when ingested at the same time as a cariogenic challenge.