Our prior studies of the molecular epidemiology of group A streptococcus (GAS) pharyngitis indicated that the most common emm types associated with pediatric pharyngitis in North America were 12, 1, 28, and 4. We previously reported that the proportions of pediatric pharyngitis due to emm types 12 and 4 decreased with increasing age throughout childhood. We hypothesized that this is due to age-associated acquisition of antibodies to the amino-terminal type-specific region of common GAS M proteins during childhood. We sought to demonstrate this in sera from healthy children by using ELISAs for M 12, 1, 28, and 4. Enzyme-linked immunosorbent assays (ELISA) using chemically synthesized peptides copying amino-terminal type-specific regions of the M proteins were performed on sera from four age groups of healthy children (group I: 3-6 years, group II: 7-10 years, group III: 11-14 years, group IV: 15-18 years). ELISA data were correlated with opsonophagocytic assays for a subset of sera and M 1 GAS. Sera from healthy 12-20-month-old children were used as negative controls. Our results showed that the highest percentage of positivity was for M12, which also showed progressive seropositivity in older children. For the other serotypes, the highest seroprevalence rates were in the 11-14-year-old age group. The presence of ELISA antibodies against M1 correlated with opsonophagocytic activity, a previously studied indicator of immunologic protection.