Nineteen human IgG myeloma proteins were tested in a sensitive assay for binding to 60 strains of β-hemolytic streptococci. Human group A, C and G streptococci demonstrated strong reactivity with positive binding of all four human IgG subclasses. Group C and G streptococci were capable of binding twice as much IgG as group A strains. β-Hemolytic bovine group G streptococci showed a low degree of reactivity with IgG1 and IgG4. Kinetic experiments revealed that the IgG binding was a time-dependent, displaceable process. Studies performed with Fab and Fc fragments showed that Fc structures are involved in the binding of intact IgG molecules to the bacterial cell surface. In addition, radiolabelied Fab fragments from two monoclonal components exhibited a definite reactivity with group G streptococci. Scatchard plots of the IgG-streptococcal interaction showed a linear section at low saturation levels. The equilibrium constants derived from such plots were 8–9 × 10 7 1./mole. The equilibrium constant for the interaction of IgG with staphylococcal protein A as measured with the Cowan I reference strain was 5 × 10 7 1./mole.