Despite uniform susceptibility of group A streptococci to penicillin, failure to eradicate group A streptococci is not uncommon in patients receiving penicillin for treatment of pharyngitis. We explored the possibility that penicillin tolerance could explain this phenomenon. We examined 48 group A streptococcal isolates from 48 patients successfully treated with penicillin (streptococci eradicated) and 92 isolates from 37 patients (one to four isolates per patient) who failed to respond to penicillin therapy (streptococci not eradicated). Penicillin tolerance was recognized by the gradient-replicate plate method and by time-kill experiments with penicillin concentrations of 16 times the minimal inhibiting concentrations. Tolerance was identified in 25% (23 of 92) of the isolates from the treatment failure group, in contrast to none of the strains from the treatment success group. Characterization of the strains by M and T typing revealed no predominant type(s) among the tolerant strains. These findings suggest that penicillin tolerance may be responsible for some instances of failure of penicillin to eradicate group A streptococci from the upper respiratory tract of individuals with streptococcal tonsillitis or pharyngitis.