Factor I is a typical multidomain protein of the complement system. It regulates complement activation by proteolytic degradation of C3b or C4b in the presence of factor H, complement receptor type 1, membrane cofactor protein or C4b-binding protein as cofactor. It is constructed from five presumed independently folded domains, namely a factor I module, a CD5-like domain, two low-density-lipoprotein receptor type A domains and a serine-proteinase domain. X-ray and neutron solution scattering was used to study the arrangement of these domains in factor I. Factor I was determined to be monomeric in solution, with an A280(1%, 1cm) of 12.3-14.1. Its radius of gyration (R(G))) was 3.96 nm by X-rays in a high positive solute-solvent contrast, and 3.84 nm by neutrons at infinite solute solvent contrast. The cross-sectional radius of gyration (R(XS)) was likewise found to be 1.64 nm by X-rays and 1.55 nm by neutrons. The R(G) data were not noticeably dependent on the solute-solvent contrast, whereas the R(xs) data showed a small dependence. The maximum dimension of factor I was determined to be 12.8 nm from the R(G) and R(XS) data, and 14-15 nm from the X-ray and neutron distance distribution functions. This length is too short to account for a linear arrangement of the domains in factor I. Small sphere models were developed for factor I in which the largest domain was modelled from the crystal structure for β-trypsin. The attachment of either an elliptical cylinder or a two-armed V-shaped structure to this domain to represent the remaining four small domains gave good scattering curve-fits for factor I, and were compatible with experimental sedimentation coefficients. The non-extended domain models for factor I imply that the steric accessibility of each domain will be reduced, and this may be important for its functional activity.