A protein responsible for clot formation was isolated from plasma of the crayfish Pacifastacus leniusculus, by repeated precipitation at low ionic strength, pH 6.0. The protein, here named clotting protein (CP), is a lipoglycoprotein, which consists of two 210-kDa subunits, covalently associated by disulfide bonds. Preparations of the CP can form stable clots in the presence of crayfish haemocyte lysate supernatant, which contains endogenous, Ca(2+)-dependent transglutaminase (TGase) activity. The covalent, TGase-mediated polymerization of CP could clearly be visualized in SDS/PAGE under reducing conditions, where the 210-kDa subunit is covalently cross-linked into dimeric, trimeric and higher polymeric forms. The CP was shown to be a substrate for transglutaminases, since two different fluorescent TGase substrates, namely dansylcadaverine and a dansylated glutamine-containing peptide, were incorporated into the CP subunit by active TGase. This indicates the presence of both glutamine and lysine residues in the CP, accessible for TGase cross-linking. The amino acid composition and the N-terminal amino acid sequence of the clotting protein was determined.