Cytologic diagnosis of reactive or malignant effusion is sometimes difficult. Especially, differentiation of benign reactive mesothelial cells from malignant cells in body effusion is more difficult. Recently, immunohistochemistry has been used to diagnose difficult cases. Phospholipase $C(PLC)-{gamma}1$ is one of the isoenzyme of the PLC which plays central role in signal transduction involving cellular growth, differentiation and transformation by phosphorylating many protein component. Increased expression of $PLC-{gamma}1$ in human breast carcinoma, colorectal carcinoma and stomach cancers are reported. To evaluate the efficacy of positive $PLC-{gamma}1$ immunostaining in the diagnosis of malignancy in effusions, paraffin-embedded cell blocks of pleural fluid and ascites from 10 patients(5 metastatic adenocarcinomas, and 5 reactive mesothelial cells) were immunostained with a monoclonal antibody to $PLC-{gamma}1$. $PLC-{gamma}1$ immuostained all the adenocarcinomas in cell block(5/5) with intense membrane pattern, however, none of the reactive mesothelial proliferations stained with the diagnostic membrane pattern. Thus, our study strongly supports the conclusion that $PLC-{gamma}1$ immunopositivity is likely to become a useful adjunct for the diagnosis of malignancy in effusions.