The production of bacteriocins by 314 Streptococcus mutans isolates from 20 mother-infant pairs was studied to test the likelihood of maternal transmission of this species. The patterns of inhibition were sufficiently different to allow differentiation of the 314 isolates into 41 bacteriocin types. The bacteriocin codes of isolates within one pair did not correspond to the codes of strains isolated from any other pair. The number of infant strains (per infant isolates) matching maternal strains within each mother-infant pair were 10/10, 10/10, 10/10, 12/12, 2/3, 10/10, 10/10, 10/10, 3/3, 5/10, 8/8, 3/3, 8/8, 3/3, 7/7, 4/4, 3/3, 8/8, 3/3 and 4/4 for pairs 1-20, respectively. Statistical analysis, utilizing a randomization test, generated a p value less than 0.0001, which is 12 standard errors above the level expected if the pairings were random.