During a prospective study of endemic group A streptococcal upper respiratory tract infection and the streptococcal carrier state, we investigated persistence of group A streptococci after treatment with recommended doses of antibiotics. We evaluated clinical findings, culture results, and streptococcal antibody responses at the acute-stage clinic visit and at convalescent-stage visits 3 and 8 weeks later. Data from 280 children ill with pharyngitis and their family contacts were analyzed (mean age 13.3 years); 129 individuals (46%) had group A beta-hemolytic streptococci isolated at the initial visit. Despite antibiotic treatment, group A streptococci were not eradicated from the upper respiratory tract of 34 (26%) of the 129 culture-positive individuals. Thirteen of the 34 (38%) required a second course of antibiotic therapy before the infecting serotype was eradicated, but a majority, the remaining 21 (62%) patients, were not bacteriologically cured despite repeated courses of antibiotics. Streptococcal antibody data suggested that the majority of the treatment failures were streptococcal carriers. Only 5% of asymptomatic individuals with a positive culture for group A streptococci demonstrated an antibody rise. These bacteriologic and serologic data document that it usually is unnecessary to retreat asymptomatic convalescent individuals having throat cultures persistently positive for group A streptococci, since these individuals seldom represent true streptococcal infection.