Sera from specimens of Negaparion brevirostris, Ginglymostoma cirratum, and Dasyatis americana were studied for hemolytic activity. Natural hemolytic activity was found in sera from all 3 species. Reactivity to heat, EDTA, hydrazine, carrageenin, and erythrocyte absorption indicated that hemolysis was mediated by a system resembling mammalian complement (C''). C'' levels of the shark sera, as measured in a modified sheep erythrocyte hemolytic system, were comparable to those of guinea pig serum. D. americana possessed low levels of hemolytic activity which also appeared to be C mediated. Elasmobranch sera were extremely labile to freezing and storage. Although considered primitive vertebrates, certain elasmobranchs appear to possess higher levels of serum complement than are found in many mammalian sera.