Phospholipase (PL) A2 is involved in signal transduction in the resistance reaction that is induced in potato by inoculation of an incompatible race of Phytophthora infestans, the late blight fungus, or by treatment with fungal elicitor hyphal wall components (Kawakita et al. 1993). In this study, PLA2 in the soluble fraction from potato tuber was purified. The following results suggested that the enzyme was, in fact, patatin: (1) the molecular mass of the purified enzyme was 40 kDa, the same as that of patatin; (2) the pI of the purified enzyme was approximately 4.75, which corresponds to that of patatin; and (3) the amino-terminal amino acid sequence of the purified enzyme showed a high degree of homology to that of patatin. Patatin is known as a storage protein of the potato tuber and it has been shown to have esterase activity. However, other enzymatic activities and the function(s) of patatin are unknown. We investigated the PLA activities of the purified patatin. The PLA2 activity of the patatin was much higher than the PLA1 activity, even though the protein exhibited both activities. The PLA2 activity of the enzyme was particularly apparent when phosphatidylcholine with linoleic acid at the sn-2 position was used as substrate. Lower activity was observed with phosphatidylcholine with palmitic acid, oleic acid and arachidonic acid at the sn-2 position.