To investigate immuno-incompetence to polysaccharide Ag in young children, antibodies to the polysaccharide and protein Ag of Streptococcus pyogenes were studied. S. pyogenes was chosen because it commonly causes natural infections and has well-characterized polysaccharide and protein Ag. In children over the age of 2 yr it was found that the maturation of antibody responses to the polysaccharide Ag of S. pyogenes (A-CHO) appeared to occur in parallel with, or even earlier, than the responses to streptococcal protein Ag. When antibodies to group A carbohydrate (A-CHO) were studied in detail, qualitative differences between the antibodies of children and adults were demonstrated. Although anti-A-CHO antibodies of adults were strikingly restricted to the IgG2 subclass, those of children were found in both the IgG1 and IgG2 subclasses. In addition, the clonal diversity of IgG antibodies to A-CHO increased with age, and additional clonotypes were detectable in convalescent sera of some subjects of all ages after infection. Two cases with major additional clonotypes after group A streptococcal infection were studied in detail. In these two cases the additional clonotypes belonged to a different IgG subclass than the previously dominant clonotypes, and the expression of the additional major clonotypes occurred in both IgG1 and IgG2 subclasses.