We have now demonstrated that activated complement component C1s cleaves beta 2-microglobulin at the position identical to that at which beta 2-microglobulin is cleaved in serum of patients suffering from lung cancer. The main cleavage is in the disulphide loop C-terminal to Lys-58, generating a modified form of beta 2-microglobulin with a two-chain structure. The C-terminal Lys-58 in the A chain is highly susceptible to removal by a carboxypeptidase-B-like activity causing the formation of des-Lys58-beta 2-microglobulin. This is the first demonstration of a noncomplement protein substrate for the proteolytic activity of C1s. The C1s-induced cleavage of beta 2-microglobulin can be inhibited in the presence of C1 esterase inhibitor, demonstrating a regulatory function of C1 esterase inhibitor in the C1s-induced cleavage of beta 2-microglobulin.