The interaction of monovalent hapten dinitrophenyl epsilon-amino caproic acid (DNP-EACA) with purified IgG1 sheep anti-DNP precipitating and non-precipitating antibodies, and their F(ab')2, F(ab') and Fab fragments, was studied by fluorescence quenching and by a radioimmunoassay. The Scatchard plots of whole non-precipitating antibody and its F(ab')2 fragment showed a bi-modal curve that could be interpreted as due to the existence of two populations of sites with very different affinity for the ligand, each population representing 50% of the total number of sites. The F(ab) fragments of the non-precipitating antibody could be fractionated by immunoadsorption into two populations of high and low affinity whose association constants differed by more than 2 logs. The study of the interaction of whole antibodies with DNP-bovine serum albumin (BSA) demonstrated that each molecule of precipitating antibody can combine with two molecules of antigen but non-precipitating antibody cannot combine with more than one molecule of antigen. It is concluded that the molecule of non-precipitating antibody is asymmetric and has a site of high affinity and another of low affinity. As a consequence of this structure the non-precipitating antibody behaves functionally as univalent and is unable to form precipitates with the multivalent antigen and to activate effector mechanisms.