Using a three-step procedure, we purified (79 and 51.6-fold to homogeneity) and characterized the two isoforms (a and b) of alpha1-proteinase inhibitor-like protein from carp seminal plasma. The isoforms have molecular masses of 55.5 and 54.0 kDa, respectively. These inhibitors formed SDS-stable complexes with cod and bovine trypsin, chymotrypsin and elastase. The thirty-three amino acids within the reactive loop SLPDTVILNRPFLVLIVEDTTKSILFMGKITNP were identified for isoform b. The same first ten amino acids were obtained for isoform a, and this sequence revealed 100% homology to carp alpha1-proteinase inhibitor (alpha1-PI) from perimeningeal fluid. Both isoforms of alpha1-PI are glycoproteins and their carbohydrate content was determined to be 12.6 and 12.1% for a and b, respectively. Our results indicated that alpha1-PI is one of the main proteins of carp seminal plasma. Using polyclonal anti-alpha1-PI antibodies, alpha1-PI was for the first time localized to the carp testis. The presence of alpha1-PI in testis lobules and in the area surrounding spermatides suggests that this inhibitor may be involved in the maintenance of testis connective tissue integrity, control of spermatogenesis or protection of tissue and spermatozoa against unwanted proteolysis. Since similar alpha1-PI has been identified in rainbow trout semen it can be suggested that the presence of alpha1-PI in seminal plasma is a common feature of cyprinid and salmonid fish.