The reversible binding of the haptens 2,4-dinitrophenyllysine (DNP-lysine) and 2,4,6-trinitrophenyllysine (TNP-lysine) to either the nitrophenyl-binding myeloma protein MOPC-315 or to pooled mouse anti-DNP or anti-TNP antibodies produces large and characteristic extrinsic Cotton effects (in the circular dichroic spectra). Despite the similarities in binding characteristics of the three proteins, the circular dichroic spectra produced by the haptens bound to the active sites of these proteins were markedly different. Extrinsic Cotton effects, therefore, provide a powerful new probe of the structure of the reversible complex formed between a hapten and an antibody active site.