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Human C4b-binding protein (C4BP) is a multimeric regulatory complement component interacting with vitamin K-dependent protein S and complement C4b. Using hybridoma technology, a panel of monoclonal antibodies (mAb) specific for intact human C4BP and its 160-kDa chymotryptic central core fragment were prepared to study the structure-function relationships of C4BP. By Western blot analysis and competition experiments, four distinct groups of mAb were identified and mapped on the C4BP molecule. By rotary shadowing, spider-like images of C4BP-antibody complexes were obtained and immunoelectron microscopy provided some information on the stoichiometry of the antibody-C4BP interaction. Certain antibodies interacted with C4BP molecules only at a ratio of 1:1. Others formed complexes of two or more antibodies bound to homologous sites on the C4BP molecule. Using an enzyme-linked immunosorbent sandwich assay for the measurement of the complex formation between protein S and C4BP, mAb against the central core and the disulfide-linked beta chain of C4BP were identified that inhibited the binding of protein S to C4BP. In a binding assay using 125I-labeled C4BP and solid-phase C4b, the inhibitory effect of one group of anti-C4BP mAb on the binding of C4BP to C4b was demonstrated.