The reagent N-bromosuccinimide (NBS) has been employed to investigate the role of tryptophan in hapten binding in anti-DNP (H-1) and anti-DNP-p-aminobenzoylglutamate (DNP-ABG) (I-13) antibodies. In 0-1 M acetate (pH 4-0) buffer fifteen and sixteen moles of tryptophan in the anti-DNP and anti-DNP-ABG antibodies respectively were reactive toward NBS. The hapten DNP-lysine protected 1 tryptophan in antibody H-1 and three tryptophans in antibody I-13 from NBS modification. DNP-ABG protected three tryptophans in antibody H-1 and five tryptophans in antibody I-13 from NBS oxidation. NBS treatment of the unprotected antibodies resulted in a significant, but not total inhibition of hapten binding, while in the hapten protected antibody preparation no significant loss of binding occurred due to NBS treatment. The binding activity of the anti-DNP antibody H-1 was more sensitive to NBS oxidation than was the anti-DNAP-ABG antibody I-13. This was apparently due to the larger number of oxidizable tryptophans in I-13 making it less sensitive to overall tryptophan modification. The results of these investigations are discussed in terms of the antibody combining site model proposed by Haselkorn et al. (Haselkorn, Friedman, Givol and Pecht, 1974) derived from kinetic mapping of the antibody-combining site by chemical relaxation spectroscopy.