The reactions between IgG and IgM anti-A and anti-B and 125I-labelled A and B blood group glycoproteins were investigated by a modification of the Farr technique. Inhibition curves obtained in a radioimmunoassay system indicate that IgM anti-A and anti-B antibodies have a significantly higher binding constant for A and B glycoproteins than IgG anti-A and anti-B. The observed difference in binding constant may explain the fact that A and B glycoproteins readily inhibit agglutination of A and B red cells by IgM anti-A and anti-B but less readily inhibit agglutination by IgG anti-A and anti-B.