The subunit composition of human C3 and C5 was analyzed. Acrylamide gel electrophoresis of the fully reduced and dissociated proteins disclosed a similar structure, consisting of one α and one β subunit, linked together by one or more disulfide bonds. The approximate molecular weights for the α and β subunits of C3 as well as C5 were 140,000 and 80,000 respectively. caused cleavage solely of C3α, whereas trypsin affected both C3α as well as C3β. A characteristic subunit modification by both enzymes indicated that C3α constitutes the source of C3a. as well as typsin exclusively affect C5α. C5a therefore appears to originate from the C5α subunit. The mode of primary cleavage by and trypsin differs, giving rise to different forms of C5b. The question is raised if multiple forms of C5a also exist. It appeared from our studies that certain forms of C5b may retain portions of the α subunit, which could potentially release some biologically active split products following secondary cleavage by the appropriate enzyme.