The cytological diagnosis of malignancy in serous effusions, at the present time, depends upon the recognition by light microscopy of morphological differences between stained malignant and non-malignant cells. Not infrequently reactive mesothelial cells present morphological changes indistinguishable from those of the 'benignlooking' malignant cells thereby presenting the cytologist with differential diagnostic problems. The distribution of the Epithelial Membrane Antigen (EMA) and the Carcinoembryonic Antigen (CEA) on cells in effusions has been examined to ascertain whether this approach is of value in discriminating between malignant and mesothelial cells. The markers were demonstrated by an indirect immunoalkaline phosphatase staining technique on smear preparations of cells from serous effusions subsequently fixed in 95% ethanol. This research was extended to include radioimmunoassay for CEA in serous effusions. A third aspect of this thesis involved an attempt to raise monoclonal antibodies to non-neoplastic mesothelial cells. Three hundred and nine routine serous effusion specimens were stained for EMA. The immunocytochemical results were correlated with the cytology of the specimens and the clinical diagnoses. The pattern of EMA staining was classified as weak or strong. Weak staining was present on non-neoplastic mesothelial cells as well as on malignant epithelial cells. Strong EMA staining was present on 63 of the 116 specimens reported as cytologically positive for malignancy. In addition, malignant cells in five cytologically suspicious and three negative effusions from patients with epithelial malignant disease were picked out by strong EMA staining. However, in three cases strong EMA staining was also observed in specimens from patients without evidence of malignancy indicating that this method may give rise to false-positive results.