Human immunoglobulin E (IgE) consists of 14 domains, each with the characteristic immunoglobulin fold structure. Compared with the 12-domain structure of immunoglobulin G (IgG), IgE has an additional pair of domains (C epsilon 2) in the Fc region in place of the hinge of IgG. The crystal structure of the 4-domain Fc fragment of IgG is known, but not that of the 6-domain Fc fragment of IgE (IgE-Fc). In order to elucidate the position of the C epsilon 2 domains in the domain structure of IgE-Fc, IgE-Fc was studied by synchrotron X-ray and pulsed neutron scattering. The upper limit on the X-ray radius of gyration RG which determines macromolecular elongation was determined to be 3.52 +/- 0.14 nm. That for the neutron RG (measured in 100% 2H2O buffers) was 3.53 +/- 0.05 nm. The X-ray and neutron cross-sectional radii of gyration were 1.89 +/- 0.05 and 1.56 +/- 0.09 nm, respectively. The scattering curves were modeled on the basis of a previously-predicted model for IgE-Fc (Helm, B. A., Ling, Y., Teale, C., Padlan, E. A., & Brüggemann, M. (1991) Eur. J. Immunol. 21, 1543-1548). The extended arrangement of domains in that model resulted in poor agreement with experimental data. Interactive and automated procedures for the fitting of crystallographically-derived domain models to scattering data were developed. Each pair of C epsilon 2, C epsilon 3, and C epsilon 4 domains was translated and rotated relative to the remaining structure in a comprehensive five-parameter search of more than 37,000 models. Substantially improved agreement between the experimental and calculated scattering curves was obtained. Bent models for IgE-Fc in which the C epsilon 2 domain pair is rotated by at least 40-50 degrees from its position in the previously predicted linear IgE model consistently gave the best agreement with the X-ray and neutron scattering curves. Such a structure for the Fc fragment accounts in part for the bent structure previously proposed for intact human IgE, which is important for understanding the interaction between IgE and its receptors.