The aim of this project was to determine the carboxyl terminal amino acid sequence of the heavy chain of human complement subcomponent c1s. The proteolytic cleavage of a peptide bond(s), probably at an Arg-Ile bond, of the single chain C1s yields the active serine protease C1s composed of a heavy and light chain. A knowledge of the amino acid sequence preceding the scissile Arg-Ile bond would allow the chemical synthesis of a model peptide substrate for the C1s-activating enzyme C1r. Human C1s was purified to homogeneity by euglobulin precipitation and repeated ion exchange chromatography. Unactivated C1s, which could be activated by incubation with partially purified C1r, was isolated by performing all purification steps in the presence of the serine protease inhibitor phenylmethane sulphonylfluoride and at low temperature. The heavy and light chains of activated C1s were separated by ion exchange chromatography in the presence of denaturant following thorough disulphide bond reduction. Isolation of the carboxyl terminal-derived peptide of the C1s heavy chain by peptide mapping as well as by chemical modification of protein carboxyl groups was unsuccessful largely due to the high molecular weight of the protein substrate. Digestion of c1s by carboxypeptidase B resulted in the very rapid release of arginine in a quantitative yield presumably from the carboxyl terminus of the C1s heavy chain. Affinity chromatography using immobilized anhydrotrypsin was successful in isolating the carboxyl terminal chymotryptic peptide of the C1s heavy chain. Anhydrotrypsin displays a remarkably specific affinity for trypsin product-like peptides possessing a carboxylterminal arginine residue. Attempts to determine the entire amino acid sequence of the isolated peptide were prevented by the difficulty in obtaining sufficient material. However, by determining the N-terminal amino acid sequence and amino acid composition of this peptide as well as by performing further peptide fragmentation by trypsin the following partial primary structure is proposed: Gln-Gln-Lys-Glx-Val-Pro-Glx-Gly- [Thr, Ser, (Leu), Ala] - Lys-Glx-Glx-Asx-Arg.