Numerous studies have reported a correlation between mutants streptococci levels and dental caries. The aim of this study was to assess the relationship between salivary mutans streptococci levels and caries in preschool children of low socioeconomic status. A total of 462 Head Start children, mean age 3.8 yr (range 2.0-5.3 yr), were examined by the modified method of Radike. Saliva samples from 458 of these children were collected with tongue blades and impressed onto mutants streptococci selective agar. Children's mutants streptococci levels were categorized as low (0 CFU), moderate (1-50 CFU) or high (> 50 CFU), and the mean dmfs was 0.40, 1.92 and 4.88, respectively. All study groups (Black, Hispanic and White) had infection rates of approximately 83%; however, 39.1% of Black children had high mutans streptococci levels compared with 28.4% of White children. Pit/fissure caries was the most prevalent disease type in children with moderate or high mutants streptococci levels, although White children in the high group had significantly less of this pattern than Blacks and Hispanics. Sensitivity, specificity, and positive and negative predictive values for the high mutants streptococci group were 91.3%, 57.5%, 69.3% and 86.3%. Results from this study indicate that differences between Black, Hispanic and White preschool children may influence caries activity within populations that have similar mutants streptococci infection levels and socioeconomic backgrounds.