The role of viridans group streptococci (Streptococcus oralis) in the prevention of colonization with nontypeable Haemophilus influenzae and Moraxella catarrhalis was investigated in an adenoid organ culture system. The adenoids from 100 patients who were undergoing adenoidectomy for either hypertrophy or recurrent otitis media were used. Streptococcus oralis Parker uniformly inhibited colonization with nontypeable H. influenzae or M. catarrhalis over a 24-hour period of incubation in adenoid organ culture. Streptococcus oralis Booth, a noninhibitory strain, did not significantly reduce colonization with nontypeable H. influenzae and M. catarrhalis. The results indicate that some strains of S. oralis may inhibit colonization with potential pathogens in the nasopharynx. It is therefore possible that colonization with inhibitory strains of viridans streptococci may be used in the nasopharynx as a relatively safe and inexpensive approach to prevention of recurrent otitis media in some children.