DEAE cellulose chromatography and immuni‐electrophoresis demonstrated immunochemical and serological differences between gamma globulin molecules with anti‐D and Anti‐Fyaspecificity. Partial neutralization of anti‐gamma globulin sera with specific subtractions of human gamma globulin showed that anti‐Fyafrequently appears to occur exclusively in the fast migrating components of gamma globulins whereas anti‐D occurs in both the “fast” and “slow” components. Other blood group systems were also studied with the same antiglobulin reagents.The antigenic determinant groups of the fast migrating components of whole gamma globulin and papain split gamma2globulin appear to be identical.The standardization of anti‐gamma globulin (Coombs) reagents is discussed in the li