Production of opacity factor (OF) by 841 pharyngeal isolates of group A streptococci from schoolchildren was studied along with T and M typing of the streptococcal strains. The majority (89%) of M typable strains were OF negative in contrast to M nontypable isolates, 45% of which were OF positive. There was no apparent relation between the production of OF by the M nontypable isolates and the type of infection (symptomatic vs. asymptomatic), degree of positivity of initial culture, antistreptolysin O response and failure to eradicate the organisms by treatment with antibiotics. However, seasonal shifts in the prevalence of M nontypable OF positive strains provided evidence of change in streptococcal ecology; such as shift would not have been detected by T typing alone. Thus, OF production is an additional epidemiologic marker that is helpful in differentiating M nontypable group A strains that bear common T antigens.