We have studied by electron microscopy a fascinating series of antidansyl monoclonal antibodies developed by Dangl et al. (Cytometry2, 395–401, 1982) which have the same variable domain but different constant domains. Three of the four subclasses of mouse IgG were represented, IgGl, IgG2a and IgG2b. Previously, Oi et al. (Nature307, 136–140, 1984) had examined the flexibilities of these antibodies by time-resolved fluorescence depolarization and found that IgG1 was least flexible, IgG2a was intermediate and IgG2b was the most flexible. In this communication we examine the conformations of circular complexes formed between these antibodies and a bivalent hapten, bis-dansyl cadaverine. The circular complexes were predominately composed of two antibodies linked into a ring by two bivalent haptens, and are referred to as dimers, since only the antibody molecules are seen with the electron microscope. A few trimers and an occasional tetramer were also present in these preparations. For the least flexible IgG1, almost all (> 99%) of the circular dimers were “open-hinge” complexes with a hinge angle between the Fab arms of 100–120°. For the intermediate IgG2a, most of the dimers were “open-hinge” complexes, but a larger percentage, 4 to 5%, had closed hinges with a hinge angle approaching 0°. For the most flexible IgG2b, over 40% of the dimers were “closed-hinge” complexes. A model is proposed to explain these differences based upon orientation of the hapten in the combining site and differences in hinge structure.