1. A series of tests were conducted to determine whether or not the hemolymph of the crayfish Astacus leptodactylus contains a plasma coagulation factor. 2. The total protein amount is higher in the plasma than in the serum. 3. Serum and plasma do not exhibit similar electrophoretic banding patterns. Plasma contains one band or a series of supplementary fractions with a high molecular weight. 4. Electrophoregrams of plasmatic clottable extract, obtained by the classical methods employed in crustacean serology, show a main fraction or a series of polymers with the same electrophoretic behavior as the additional fractions seen on the plasma pattern. 5. This solution clots when treated with CaCl2 and a cellular extract. 6. Immunoelectrophoresis demonstrates the presence of a clottable protein precipitate line in plasma, but this protein also gives a very faint similar line in serum.