Purified IgG1, IgG2, IgG3, and IgG4 myeloma proteins were coupled to red blood cells with chromic chloride either individually or in various combinations. Only red blood cells coated with IgG1 or IgG3 formed rosettes; red cells coated with IgG3 appeared to predominate in this reaction. IgG2, IgG4, and IgA myeloma proteins coupled to red blood cells did not appear to inhibit the IgG1 and/or IgG3 also on the red cells from forming rosettes. The IgG subclass specificity of a number of red cell isoantibodies was determined. The majority were IgG1 and IgG3, as previously noted. IgG2 was observed among some of these antibodies, namely in anti-D, anti-CD, anti-CDE, anti-s, and anti-Jka. Although rosette formation was strongly associated with the relative amounts of IgG1 and IgG3, particularly the latter, on red cells, a number of exceptions were noted.