We examined the immunohistochemical expression of membrane-associated phospholipase A(2) (M-PLA(2)), belonging to group II PLA(2), in 44 advanced gastric cancers, using the ABC method and monoclonal antibody anti-human M-PLA(2). M-PLA(2) mRNA was also examined in the same rumours by Northern blot analysis. In addition, the content of M-PLA(2) protein and prostaglandin E(2) (PGE(2)) in the malignant lesion and in the non-malignant gastric mucosa was examined. The expression was detected in cancer cells in 31 out of 44 advanced gastric cancer tissues (70.4%) by the ABC method. M-PLA(2) mRNA was detected in 36 out of 44 gastric cancer tissues (81.8%), and the density was observed to be higher in tumour tissues than in the adjacent nonmalignant gastric mucosa. The M-PLA(2) protein was detected both in malignant tissues and in non-malignant gastric mucosa, and the content of M-PLA(2) protein was significantly higher in malignant tissues than in the non-malignant gastric mucosa. There was a significant positive correlation between the expression of M-PLA(2) mRNA and the amount of M-PLA(2) protein. PGE, was also detected in the malignant tissues and in the non-malignant mucosa. The content of PGE, was significantly higher in the former. These results indicate that M-PLA(2) is produced both in malignant and nonmalignant cells of the stomach, the former producing higher amounts of this enzyme than the latter. M-PLA(2) may be involved in cancer progression through its function or through the function of products of this enzyme's action such as PGE(2) in gastric cancer.