C3c and C3d fragments were prepared in pure form from trypsin-digested human C3, and the individual chains of tryptic C3c were isolated by gel filtration on Sepharose 4B in 6M guanidinium hydrochloride. No low mol. wt (Mr) fragments were identified. The polypeptide chains were characterized with regard to Mr, amino acid composition and N-terminal amino acid sequence. Tryptic C3c consisted of one fragment from the beta-chain (Mr 64,000) and two from the alpha'-chain (Mr 40,000 and 23,000). The beta-chain fragment was derived from the C-terminal part of the chain, and the 23,000-Mr component constituted the amino terminal end of the alpha-chain. The 40,000-Mr fragment emanated from the C-terminal end of the alpha-chain. Tryptic C3d displayed microheterogeneity on polyacrylamide gel electrophoresis in sodium dodecyl sulfate, but possessed a homogeneous N-terminal, identical to that described by Tack et al. (1980) (Proc. natn. Acad. Sci. U.S.A. 77, 5764-5768). By utilization of antisera against subunits of C3 and C3c in immunoblotting a degradation scheme for C3 by trypsin was proposed and the positions of the fragments in the intact molecule indicated.