Protein kinase C (PKC) is believed to play an important role in tumorigenesis, so we measured PKC activities of human gastric carcinomas and adjacent normal mucosae to elucidate its role for gastric carcinogenesis using PegTagtrade mark non-radioactive protein kinase C assay. The mean activities of microsomal PKC in carcinoma and normal mucosa were 449+/-179, 661+/-264 pmol/min/mg, respectively (p=0.001). Cancerous microsomal PKC activity decreased when the tumor was small, node-metastasis was negative, the depth of invasion was shallow and the histological type was differentiated. Our results suggested that PKC activity would be down-regulated in membrane of the early stage of gastric carcinoma.