The specificity of the IgM receptor expressed by human T cells cultured in IgM-free media has been investigated. IgM receptors have been detected using a rosette system with ox erythrocytes coated with rabbit antibody (EA(IgM)), and the inhibitory capacity of different IgM fragments in the rosette system has been tested. It was found that F(c)5mu but not F(ab')2mu, nor monomeric IgM (8 S IgM) inhibited EA (IgM) rosette-forming cells. This indicates that the receptor present on the surface of T cells has affinity for a structure located in the Fc portion of the pentameric IgM.