Increased immune precipitation of purified chicken anti-DNP antibody with homologous antigen at physiological salt concentrations was measured by lowering the pH to 5·0. Ultracentrifuge studies indicated that at pH 5·0 the antibody molecule was not aggregated and existed as a monomer with a S 20, w of 6·75 and a molecular weight of 172, 600. Electrofocusing profiles of chicken anti-DNP antibody gave a pI value of 6·6 for the monomeric form. Binding studies of chicken anti-DNP by fluorescence quenching indicated that pH 5·0 had little or no effect on the direct interaction between antigen and antibody. Equilibrium dialysis studies with 3H-∈-DNP- l-lysine at pH 5·0 gave an average intrinsic association constant of 1·7 × 10 6 M −1 identical to that at pH 8·0. The number of ligand binding sites per antibody molecule, based on a molecule of 172, 600 was 2·0.