Failure of Farr test in detecting IgM response to dinitrophenyl in mice

In mouse serum the ammonium sulphate technique does not detect the immune response to the haptenic determinant DNP on polysaccharide, nor the early stage of the primary response to DNP on protein. Specific anti-DNP antibodies are easily detected, in the same sera, by the phage neutralization technique. This failure of the Farr test seems to be related to its inability to measure adequately mouse anti-DNP 19S antibodies.