Plaque samples from caries lesions, white spot margins of these lesions, and caries-free smooth enamel surfaces, obtained from seven subjects with nursing bottle caries, all contained unusually high proportions of Streptococcus mutans. The proportion of lactobacilli present in plaque from caries lesions was statistically greater (p less than 0.01) than the proportion present in plaque obtained from white spot margins of these lesions or caries-free smooth enamel surfaces.