The cytomorphologic and cytochemical investigation of 59 samples of pleural and peritoneal effusions with malignant tumor cells was performed and the results were compared. The diagnosis was confirmed histologically in most patients. The cytomorphologic method gave good results not only in the determination of malignant tumor cells but also in the differentiation of various kinds of tumors. A diagnosis was made of differentiated carcinoma (adenocarcinoma and squamous carcinoma) in 40 cases, undifferentiated carcinoma in 5, sarcoma or hemoblastosis in 9 and undifferentiated malignant tumor in 5 cases. The results of cytochemical reactions investigated demonstrated a significant difference between the cells of carcinoma and sarcoma, and in various kinds of carcinoma the most important difference was found between the positive reaction of alkaline phosphatase and negative reaction of acid phosphatase in the cells of ovarian and uterine carcinoma. Although the morphologic examination of effusions was considered most important for the diagnosis of malignancy, cytochemical methods can sometimes be helpful in a more precise differentiation.