The quantitative bacteriology of the adenoid was studied in 34 otitis-prone and 25 non-otitis prone children. Viridans streptococci appeared to be the predominant normal flora in children who are non-otitis prone. There was a significant decrease in viridans streptococci in the otitis-prone child compared to the non-otitis-prone child. There was a significant increase in nontypable Haemophilus influenzae (NTHI) in the otitis-prone child. The mechanisms responsible for this alteration of the microecology of bacteria of the nasopharynx may be related, in part, to bacterial interference or to the inappropriate use and over-use of antibiotics. In vitro inhibition of growth of NTHI was demonstrated with selective strains of viridans streptococci. A preliminary analysis of an inhibitory strain and a non-inhibitory strain of viridans streptococci are presented and their biochemical profiles and antibiotic sensitivities were entirely different. A possible mechanism for the inhibition on NTHI by viridans streptococci has been suggested. This mechanism may be related to an alteration of pH in the growth media or the possibility of the utilization of nutrients required for growth of NTHI.