A mixture of rabbit IgM anti‐human A1red cell antibody and a human125I‐labelled monoclonal IgM was reduced with mercaptoethanot and exposed to oxygen under conditions in which most of the half subunits coalesced to IgM subunits with reformation of interchain disulphide bonds. Hybrid molecules were demonstrated in the mixture by precipitin tests with autoradiography and by binding of125l to human A1red cells. By careful selection of IgM antibodies with high energy in each binding site, such hybrids may be useful ‘IgM coats’ for haemagglutination inhibition tests attempting to define sub‐class‐specific and genetically determined anti