Bacterial immunoglobulin-binding proteins expressed on the surface of group A streptococci represent a heterogeneous family of functionally related proteins. In this report, we describe efficient methods for extracting immunoglobulin-binding proteins and classifying them functionally and antigenically. A common characteristic of immunoglobulin-binding proteins expressed by group A streptococci appears to be the absence of internal methionine residues in the binding protein. This has enabled development of a rapid, efficient, cyanogen bromide-based extraction procedure for solubilizing these molecules from intact bacteria. Studies carried out with a series of monospecific polyclonal antibodies prepared in chickens have identified two major antigenic classes of immunoglobulin-binding proteins. The methods described in this report facilitate a rapid functional and serological screening of immunoglobulin-binding proteins that should now enable detailed epidemiological studies of the importance of these molecules in group A streptococcal infections and their relationship to other surface proteins, in particular, the antiphagocytic M protein.