Solution X-ray-scattering experiments with the use of synchroton radiation on the human complement-component-C4b-binding protein showed that its R(G) is 13 nm and that its M(r) is 550,000. From the known primary amino acid sequence and estimated carbohydrate content, C4b-binding protein is inferred to have a total of 7.4 ± 1 subunits. Heptameric computer models for C4b-binding protein were based on the X-ray-scattering curve to a resolution of 6.4 nm, and literature values for sedimentation coefficients and electron-microscopy images. The macromolecule was represented by a bundle of seven arms held together at the C-terminal end and spaced out by a base containing 23% of C4b-binding protein by volume. If the overall length of each arm is assumed to be 33 nm as seen in electron microscopy, the solution data indicate an average arm-axis angle of 5-10° . The seven arms of C4b-binding protein are found to be close together, in distinction to the splayed-out images seen in electron micrographs.