Some urodele amphibians possess the capacity to regenerate their body parts, including the limbs and the lens of the eye. The molecular pathway(s) involved in urodele regeneration are largely unknown. We have previously suggested that complement may participate in limb regeneration in axolotls. To further define its role in the regenerative process, we have examined the pattern of distribution and spatiotemporal expression of two key components, C3 and C5, during limb and lens regeneration in the newt Notophthalmus viridescens. First, we have cloned newt cDNAs encoding C3 and C5 and have generated Abs specifically recognizing these molecules. Using these newt-specific probes, we have found by in situ hybridization and immunohistochemical analysis that these molecules are expressed during both limb and lens regeneration, but not in the normal limb and lens. The C3 and C5 proteins were expressed in a complementary fashion during limb regeneration, with C3 being expressed mainly in the blastema and C5 exclusively in the wound epithelium. Similarly, during the process of lens regeneration, C3 was detected in the iris and cornea, while C5 was present in the regenerating lens vesicle as well as the cornea. The distinct expression profile of complement proteins in regenerative tissues of the urodele lens and limb supports a nonimmunologic function of complement in tissue regeneration and constitutes the first systematic effort to dissect its involvement in regenerative processes of lower vertebrate species.