The binding of fifteen IgA myeloma proteins to protein A was studied using affinity chromatography on protein A-Sepharose CL-4B. The percentage to which the proteins bound was variable from 1% to 11% with the exception of IgA(GED) with a binding capacity of 22%, and IgA(KLO) with a binding capacity of 84%. The binding of the proteins IgA(GED) and IgA(KLO) was studied further. IgA(GED) was a monomer, Iga(KLO) a polymer. The characteristics of the binding of these proteins were different. The protein A-reactive fraction of IgA(KLO), but not of IgA(GED) remained fully reactive on repeated protein A chromatography. Furthermore, the binding of IgA(GED) could be reduced to about 3% by either a decrease in pH to 4.5 or an increase in NaCl concentration to 2.0M, whereas the binding of IgA(KLO) was similarly reduced by a decrease in pH but its binding only reduced to half of the original value on a similar increase of NaCl concentration. In addition, F(ab')2 fragments of IgA(KLO), but not of IgA(GED), bind to protein A-Sepharose CL-4B, whereas IgA1 protease-produced Fab fragments of neither of the proteins did so, nor did pepsin-produced Fab' fragments. This suggests that the binding of F(ab')2 fragments of IgA(KLO) needs an intact hinge region.