The purpose of this study was to investigate infant oral mutans streptococci (MS) by feeding method and by the mother's oral MS status. For this cross-sectional data collection, full-term infants at 4-6 weeks of age and their mothers were tested for oral MS using the Dentocult(®) SM chairside test (Orion Diagnostica, Espoo, Finland). For the 104 mother-infant pairs (68 breastfed, 36 formula-fed), the two groups differed by mother's race/ethnicity (p=0.006) but not by delivery mode, level of care at birth, or MS in the mothers or the infants. Thirty-one percent of mothers and 12% of infants were MS positive. Infant MS was not predicted by mother's race/ethnicity, delivery mode, feeding method, or MS status. When stratified by feeding method, the mother's MS status did significantly predict the infant's MS status in the breastfed group (odds ratio=5.97; 95% confidence interval, 1.06-33.7; p=0.043). In conclusion, oral MS at >10(4) colony-forming units/mL were detected in 12% of the 4-6-week-old infants, and the effect of the mothers' oral MS status was modified by the feeding method. Future research is needed to clarify the modifiable perinatal and maternal risks associated with the development of the infant's predentate oral microbial environment, before the introduction of the first tooth.