The colonization of mucosa cells in the oral cavity of newborn infants was studied at various intervals after birth in an attempt to define the nature of the epithelial binding sites for group A streptococci and their lipoteichoic acid (LTA). Stained smears of buccal mucosa cells showed that the average number of naturally acquired bacteria/cell was zero to one in infants less than one day old, one to four in infants one day old, and 11-19 in infants two days old. Samples of the same mucosa cells were incubated with group A streptococci, and the average number of streptococci bound per cell was 10-31 in infants less than one day old, 33-62 in one day old infants, and 75-100 in two day old infants. Experiments that were repeated with group B streptococci type III produced similar results. LTA, the substance that mediates the binding of streptococci to epithelial cells, was similarly bound to fewer buccal mucosal cells obtained within 6 hr of birth than cells obtained during the next 48 hr. Streptococcal and LTA binding reached adult levels between 48 and 72 hr after birth. No difference was shown in the streptococcal binding capacity of oral epithelial cells obtained from mothers at term and cells obtained from other normal adults. Preincubation of adult buccal cells with amniotic fluid did not decrease streptococcal binding. These studies demonstrated a scant capacity of the oral mucosa cells of neonates to bind LTA and streptococci and suggest that LTA binding sites are developed or unmasked during the first few days after birth.