A panel of seven monoclonal antibodies was applied to smears of cell deposit from 70 pleural and peritoneal fluids, using an immunoalkaline phosphatase (IAP) procedure. The cases were chosen to show typical cytological patterns, both benign and malignant, and in this way the diagnostic value of the method could be assessed. The antibodies used were 2D1 (anti-leucocyte), Ca 1, HMFG-2 (anti-milk fat globule membrane), LE61 and M73 (both anti-intermediate filament antibodies), anti-CEA, and K92 (anti-keratin). The anti-leucocyte antibody was found useful for distinguishing lymphoma from carcinoma. Anti-CEA gave positive reactions in 80% of carcinoma cases and was not seen to react with any other cell types. Ca 1 was positive with some cells in 95% of carcinoma cases, but mesothelial cells reacted with it in two cases. A strong reaction with the anti-milk fat globule membrane antibody was very constant in carcinoma but was also seen in mesothelial cells in 30% of benign effusions. The anti-keratin reacted with malignant cells in only a small proportion of cases. The antibodies against epithelial intermediate filaments reacted equally strongly with benign mesothelial cells and carcinoma cells, but gave negative reactions with lymphoma cells. It is concluded that a suitably chosen panel of monoclonal antibodies can be of great value in identifying neoplastic cells in serous effusions.