The transmission of mutans streptococci is thought to occur along familial lines based on investigations which have shown common strains within family units by using phenotypic typing methods, such as bacteriocin production and immunity profiles and serotyping. Difficulties in implementing these typing methods, coupled with conflicting interpretation of results between laboratories, led us to study the conservation of Streptococcus mutans strains within a mother-child cohort by using a genotypic marker, plasmid DNA. Plasmids (all 5.6 kilobases in size) were observed at an overall frequency of 3.3%, with a significantly different frequency in whites (1.5%) compared with blacks (6.6%). Plasmid-containing strains were significantly clustered in mother-child pairs compared with nonrelated individuals (58 versus 3.3%; P < 0.001). Moreover, the different plasmid groups (I and II) were highly conserved within racial boundaries (P = 0.007). In those instances in which we were able to characterize distinct strains by either biotype or plasmid profile, we found that mothers harbored a more heterogeneous population of mutans streptococci than did their children. This suggested, among other possibilities, that children acquire additional strains as they approach adulthood. Alternatively, we may have been unable to detect more heterogeneity of strains in children because of quantitative differences of strains in saliva. We present collective data which show that strains of S. mutans are highly conserved within not only mothers and their children but also racial groups, suggesting vertical transmission of this organism within human populations. Moreover, we show that levels of S. mutans found in the saliva of the mother correlated with levels found in her child, demonstrating a quantitative relationship within mother-child pairs.