A number of group A streptococcal isolates have been compared for their nonimmune reactivity with each human IgG subclass, and rabbit, pig, or horse IgG. The results obtained demonstrate considerable heterogeneity in the expression of type II IgG-binding proteins among and within group A isolates. Extraction and analysis of type II IgG-binding proteins from selected strains demonstrate the existence of five functionally distinct IgG-binding proteins. The type IIo IgG binding protein displayed the greatest range of reactivities, binding to all four human IgG subclasses, and rabbit, pig, and horse IgG. A variant of this protein, designated type II'o, bound all four human subclasses and rabbit IgG, but failed to react with pig or horse IgG. A type IIa protein was recovered from certain group A strains which bound human IgG1, IgG2, IgG4, as well as reacting with rabbit, pig, and horse IgG. A functionally related type IIc activity that displayed all of the reactivities of the type IIa protein but did not bind with human IgG2 was also identified. The final functional form of group A IgG-binding protein, the type IIb protein, bound exclusively to human IgG3. Comparison of these functionally different type II IgG-binding proteins demonstrated no simple structure-function relationship. These studies underscore the heterogeneity of type II Ig-binding proteins expressed by different group A streptococci and document that a single strain can change its pattern of expression of type II IgG-binding protein both quantitatively and qualitatively.