1. Crude salivary gland extract of the giant Amazon leech, Haementeria ghilianii, contains an inhibitor of plasma factor XIIIa. 2. The inhibitory agent was purified to homogeneity by anion-exchange, cation-exchange, gel-filtration and reverse-phase chromatography to yield a single band on SDS/PAGE with an apparent molecular mass of 7.3 kDa. It has been named tridegin. 3. Micro-sequencing of proteolytic fragments showed tridegin to be a peptide of 66 amino acids. The sequence is unique with little similarity to other leech-derived proteins. 4. Inhibition of plasma factor XIIIa activity was confirmed by four independent methods: tridegin increased the solubility of fibrin clots in urea, inhibited ammonia produced from the incorporation of ethylamine into casein, inhibited the incorporation of 5'-(biotinamido)pentylamine into casein and prevented gamma-dimer formation in clotting fibrinogen. 5. The IC50 of tridegin (approx. 9.2 nM) is very close to the concentration of factor XIIIa used in the assay and in fact depends on its concentration. This is the most potent inhibitor of factor XIIIa yet described. 6. Tridegin also inhibits platelet factor XIIIa (factor XIIIAa) with a similar potency to that of the plasma enzyme. 7. Tridegin also inhibits tissue transglutaminase but with lower potency and independently of the enzyme concentration. 8. Tridegin appears to be specific for transglutaminases, since it has no effect on the coagulation times of human plasma, on thrombin or factor Xa. Moreover it has no effect on other thiol-containing enzymes and has no ability to digest fibrinogen or cleave the isopeptide substrate, L-gamma-glutamyl-4-nitroanilide.