IgM from 7 sera of patients with macroglobulinemia has been examined for interaction with staphylococcal protein A. Three of the IgM proteins were fixed to a protein A-Sepharose column. They gave a direct co-precipitation and the “star-phenomenon”, but no direct precipitation in agar. The primary protein A reactive sites were localized to the Fc-region, but co-precipitation and “star”-formation were also dependent on other molecular configurations. Two of the sera examined contained both protein A reactive and protein A non-reactive IgM in different ratio.