The surface of the syncytial epithelium of the human parasite, Schistosoma mansoni, consists of an apical plasma membrane (APM) and an overlying envelope (En). The rapid turnover of these membranes is an adaptation to parasitism and is influenced by ambient signals emanating from the host's immune system. The third component of complement (C3) has been shown to stimulate the synthesis of the En via a C3-binding protein (C3bp) and a Ca(2+)-dependent signal transduction mechanism. Using ELISA the C3bp was found to be restricted to the En. In addition, cross-linking of iodinated C3 to En proteins with the homobifunctional noncleavable disuccinimidyl suberate reagent yielded a receptor-C3 complex in excess of 250 kDa, and SDS-PAGE analysis of solubilized En proteins that were radiolabeled and chromatographed on C3-Sepharose revealed a 130-kDa protein that specifically bound to the C3 beads. In further experiments, using a photoactivatable radiolabeled cross-linker, the Denny-Jaffe reagent, C3 transferred the radiolabel to a 130-kDa En protein. Metabolic labeling experiments have demonstrated that this C3bp is synthesized by the parasite and, more importantly, antibodies raised against the C3bp blocked En synthesis in vivo. Also, the surface localization of the C3bp was demonstrated using immunolabeling electron microscopy. The data presented herein strongly suggest that the 130-kDa schistosome En protein is a C3bp responsible for renewal of the En in response to C3 binding.