Factor H (FH) is a major regulator of complement alternative pathway activation. It is composed of 20 short complement regulator (SCR) domains and is genetically associated as a risk factor for age-related macular degeneration. Previous studies on FH suggested that it existed in monomeric and dimeric forms. Improved X-ray scattering and analytical ultracentrifugation methodology for wild-type FH permitted a clarification of these oligomeric properties. Data at lower concentrations revealed a dependence of the X-ray radius of gyration values on concentration that corresponded to the weak self-association of FH. Global sedimentation equilibrium fits indicated that a monomer-dimer equilibrium best described the data up to 1.3 mg/ml with a fitted dissociation constant K(D) of 28 microM and that higher oligomers formed at increased concentrations. The K(D) showed that about 85-95% of serum FH will be monomeric in the absence of other factors. Size-distribution analyses in sedimentation velocity experiments showed that monomeric FH was the major species but that as many as six oligomeric forms co-existed with it. The data were explained in terms of two weak dimerisation sites recently identified in the SCR-6/8 and SCR-16/20 fragments of FH with similar K(D) values. These observations indicate a mechanism for the progressive self-association of FH and may be relevant for complement regulation and the formation of drusen deposits that are associated with age-related macular degeneration.