The seminal plasma of teleost fish contains a species-specific system of proteins with anti-trypsin activity. Serine proteinase inhibitors are one of the main proteins of fish seminal plasma and their electrophoretic pattern is similar to blood plasma. Anti-trypsin activity correlates with protein and sperm concentration. To date one of the proteinase inhibitors of seminal plasma of common carp and rainbow trout has been identified. It belongs to the serpin family and is similar to α₁ -antiproteinase. A characteristic feature of this inhibitor is the ability to form an irreversible complex with serine proteinases. Serine proteinase inhibitors of fish seminal plasma may participate in the protection of reproductive tissue and spermatozoa from proteolytic attack.