The complement proteins constitute a family of large multidomain proteins with generally unknown solution structures. Solution scattering offers a powerful means of determining these solution structures, particularly when combined with the results from electron microscopy and/or analytical ultracentrifugation and also with known atomic structures for individual domains in these proteins. This method has been applied to 13 of the 18 soluble complement proteins. While many of them have highly extended structures, others have relatively compact structures. To facilitate studies of the multidomain serine proteases of complement, scattering analyses of β-trypsin and α-chymotrypsin and their proenzymes were carried out, and their scattering properties were well-reproduced by reference to their crystal structures. Factor I has a five-domain structure that includes a serine protease-like domain. Its overall length of 13 nm from scattering is too short to allow for a highly extended solution structure of domains, and scattering curve fits were best achieved by arranging these domains into a more compact form. In terms of the functional role of factor I, its domains appear to have a restricted surface accessibility for interactions with its substrates.