From the coagulocytes (amoebocytes) coagulogen (fibrinogen) was isolated, and purified on Sephacryl S-200. The cell homogenate contained one major protein species with a minimum molecular weight of 70,000. This protein clotted in the presence of human thrombin, human factor XIII and Ca++. The coagulogen contained no free thiol groups, however these were detectable in the reduced protein. Using the cyanoethylation procedure, it was estimated that one coagulogen molecule contained two lysine residues which participated in the cross-linking reaction. The total amino acid composition of the crab coagulogen and coagulin (fibrin) was estimated and compared with the amino acid composition of the Limulus polyphemus, lobster, (Panulirus interruptus) and porcine fibrinogen.