We have recently shown that Sparus aurata, the gilthead sea bream (a diploid species), similarly to rainbow trout (a quasi-tetraploid species), possesses multiple forms of the third form of complement (C3). In the present study we have evaluated the ability of the gilthead sea bream proteins to function as active C3 molecules. All five C3 isoforms could be fixed covalently to sheep erythrocyte ghosts and were able to bind to various complement-activating surfaces in the presence of MgEGTA. In the absence of MgEGTA their binding capacities generally increased, presumably as a result of classical-pathway activation by the natural antibodies present in the serum. The presence of EDTA abrogated the binding of all C3 isoforms to the various surfaces tested. The C3 isoforms differed in the efficiency of their binding to complement-activating surfaces: the two most abundant C3 isoforms (C3-1 and C3-2) bound to zymosan as well as to sheep and rabbit erythrocyte ghosts, whereas C3-3, C3-4 and C3-5 were unable to bind to zymosan. Upon complement activation, all five C3 isoforms were cleaved to 'iC3b' by factor H and I-like proteins, generating fragments similar to those generated from C3 molecules of other species. Furthermore the degradation of methylamine-hydrolysed C3 isoforms to iC3b was significantly inhibited by EDTA. The structural and functional diversity that we have observed in the C3 isoforms of S. aurata would increase the capacity of this fish to recognize a broader spectrum of potential pathogens and reinforce a specific immune response, which in fish is delayed compared with that of higher vertebrates, and is based on a single Ig type (IgM).