Magnetic resonance techniques are used to refine the model of the combining site of the Fv fragment of the dinitrophenyl binding mouse myeloma protein MOPC 315 constructed by Padlan, Davies, Pecht, Givol & Wright (1976) (Cold Spring Harbor Symp. Quant. Biol. 41, in the press). Light absorption studies indicate a dinitrophenyl tryptophan interaction in the Fv fragment of the type occurring in free solution. The Dnp aspartate tryptophan complex is therefore used as a starting point for the n.m.r. (nuclear magnetic resonance) analysis of the dinitrophenyl Fv fragment interaction. Ring current calculations are used to determine the geometry of the complex. The specificity of complex formation between dinitrophenyl and tryptophan is confirmed by the lack of ring current shifts of the dinitrophenyl resonances when tryptophan is replaced by any other aromatic amino acid. Proton n.m.r. difference spectra (at 270 MHz), resulting from the addition of a variety of haptens to the Fv fragment, show that the combining site is highly aromatic in nature. Calculations on the basis of ring current shifts define the geometry of the combining site, which involves a dinitrophenyl ring in van der Waals contact with 4 aromatic amino acid residues on the protein. The observation of a nuclear Overhauser effect on the H(3) resonance of the dinitrophenyl ring provides additional constraints on the relative geometry of the H(3) proton and an aromatic amino acid residue on the Fv fragment. The specificity of the Fv fragment for dinitrophenyl ligands arises from a stacking interaction of the dinitrophenyl ring with tryptophan 93(L), in an 'aromatic box' of essentially tryptophan 93(L), phenylalanine 34(H) and tyrosine 34(L); asparagine 36(L) and tyrosine 34(L) also contribute by forming hydrogen bonds with the nitro groups on the dinitrophenyl ring. The n.m.r. results also confirm that the antibody hapten reaction may be visualized as a single encounter step. An Appendix shows the method of calculation of ring currents for the 4 aromatic amino acids and their use in calculating structures.