The clottable protein (CP) was isolated from white shrimp, Penaeus vannamei plasma as a 400-kDa protein that splits to two identical 200-kDa subunits when it is reduced with 2-ME. However, using DTT as reducing agent, four main bands were observed; two of them (179 and 125 kDa) had the same N-terminus sequence of the intact CP, indicating that most fragmentation occurs in the carboxy-terminus. The proteinase activity of reduced CP was detected using azoalbumin as substrate. Proteinase activity was only detected in the reduced, but not alkylated protein. Trypsin and papain, as well as soybean trypsin inhibitor and E64, were included for comparison. Proteolytic activity of reduced CP was inhibited by E64, but not by STI, indicating that such activity corresponds to a cysteine type proteinase.