Microbiological studies of the core of tonsils removed from children with recurrent tonsillitis due to group A beta-hemolytic streptococci were conducted during three periods, with 50 patients in each period: 1977-1978 (period 1), 1984-1985 (period 2), and 1992-1993 (period 3). Mixed flora were present in all tonsils, with 8.1 organisms per tonsil (3.8 aerobes and 4.3 anaerobes). The predominant isolates in each period were Staphylococcus aureus, Moraxella catarrhalis, Peptostreptococcus species, pigmented Prevotella species, Porphyromonas species, and Fusobacterium species. The rate of recovery of Haemophilus influenzae type b increased from 24% in period 1 to 76% in period 2 (P < .001); a decline to 12% in period 3 correlated with a concomitant increase in the frequency of recovery of non-type b strains of H. influenzae from 4% and 10% in periods 1 and 2, respectively, to 64% in period 3 (P < .001). Both the rate of recovery of beta-lactamase-producing bacteria and the number of these organisms per tonsil increased over time. Specifically, beta-lactamase-producing strains were detected in 37 tonsils (74%) during period 1, in 46 tonsils (92%) during period 2, and in 47 tonsils (94%) during period 3, and the number of such strains per tonsil increased from 1.1 in period 1 to 2.9 and 3.3 in periods 2 and 3, respectively.