SP-40,40 is a two-chain serum protein which acts in vitro as a potent inhibitor of the assembly of the membrane attack complex of human complement. I t contains I 0 cysteine resid ues, the numbers and locations of which are conserved in several mammalian species. Evidence is presented that all the cysteine residues are involved in interchain (~--~) disulphide bonds. There are no fr~ cystcine residues. The disulphide bond motif estal~lished in this study for SP.40,40 is unique and bears no obvious homology to those complement components whose disulphide bonds have been assigned, nor is there any homology apparent between SP-40040 and other multi-chain proteins containing disulphide bonds.

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