Coombs’ test was performed with tanned red cells and 200 human sera. The titres varied between 64 and 1,024. Isolated IgG gave agglutination in corresponding dilutions. IgM agglutinated tanned red cells directly, but the reaction was inhibited by IgG. Fc-fragments of IgG gave agglutination in Coombs’ test with tanned cells while F(ab′)2-fragments did not. Accordingly, the binding of normal serum to tanned red cells depends on the Fc-part of the IgG molecule. Coombs’ test can be performed with tanned red cells employing pepsin-digested whole serum.