The gene of the most abundant protease inhibitor in potato cv. Elkana was isolated and sequenced. The deduced amino acid sequence of this gene showed 98% identity with potato serine protease inhibitor (PSPI), a member of the Kunitz family. Therefore, the most abundant protease inhibitor was considered to be one of the isoforms of PSPI. The PSPI group represents approximately 22% of the total amount of proteins in potato cv. Elkana and is composed of seven different isoforms that slightly differ in isoelectric point. Antibodies were raised against the two most abundant isoforms of PSPI. The binding of these antibodies to PSPI isoforms and protease inhibitors from different groups of protease inhibitor in potato showed that approximately 70% of the protease inhibitors present in potato juice belong to the Kunitz family.