The binding of murine IgG1, IgG2a, IgG2b, IgG3 protein molecules to Staphylococcal A protein is presented. Differential elution by sodium thiocyanate gradients is described. Proteins of the IgG2 class (either a or b subclass) require between 1.5 and 2.0 M for complete elution, whereas, IgG1 proteins are fully eluted with only 0.5 M NaSCN. These differential elution patterns can be utilized to distinguish between, or prepare proteins of different Ig classes. It is proposed that this quantitative rather than qualitative distinction between Ig subclass binding to Staph A, indicates the existence of multiple binding sites per molecule, with different markers of such sites being present on the heavy chain of the different Ig classes.