The C3 molecule is crucial in the function of the complement system. It is the only substrate for both the classical and the alternative pathways. Native C3 exhibits no biological activity. Upon activation, biologically active C3 fragments are formed, on which new antigenic determinants appear. Activated C3 express distinct binding sites for other complement proteins and cell surface receptors. Recently, it has been possible to study these neoepitopes with monoclonal antibodies. These antibodies are also found valuable in the detection and quantitation of C3 activation products and C3-containing circulating immune complexes.