The interaction of rabbit antibodies against three protein antigens differing widely in their acid-base properties was investigated by the quantitative precipitin titration. The effect of pH in the range 4-10 on these antigens, namely mannose/glucose specific Cajanus cajan lectin, human immunoglobulin G and hen egg white lysozyme, was investigated at the equivalence zone molar ratios. In each case, the extent of reaction was maximal near neutral pH and diminished by changing the pH below or above neutral. The pH precipitation profiles were similar and analysis of their respective pK values suggested that the same set of ionisable groups are involved in antibody-antigen interactions regardless of the nature of protein antigen. These are the critical ionisable groups contributed mainly by the antibody molecule.