A commercial Rho (D) immune globulin after heating at 63 C became Ripley‐like in the Rh‐positive red blood cells coated with this heated globulin carried biologic activities similar to those of red blood cells coated with Ripley anti‐CD serum. These coated red blood cells fixed complement and were agglutinated by all 20 sera containing rheumatoid factor (RF). The RF‐Rh‐hemagglutinations were more readily inhibited by heated than by unheated human IgG. The heated globulin had no such effect on Rh‐negative red blood cells. Fractionation studies by Na2SO4 precipitation and/or Sephadex G‐200 gel filtration revealed that heat‐induced IgG aggregates in heated globulin were responsible for the biological activities. In contrast, these activities in Ripley serum were carried by IgG monomers. Another anti‐CD serum (Heyman), tested in paralledl, was found to be indistinguishable from Ripley. a pooled RF serum, after multiple adsorptions with red blood cells coated with globulin, lost its agglutination activity to red blood cells coated with Riple