The use of tumour associated antigens in the diagnosis of serous effusions was studied in 76 patients with benign and 200 patients with malignant disease. Tissue polypeptide antigen (TPA), alpha fetoprotein, and CA 125 were found to be of little value. At cut off points of 3 ng/ml, 10 U/ml, and 30 U/ml, respectively, carcinoembryonic antigen (CEA), biliary glycoprotein I (BGP I), and CA 19-9 discriminated between benign and malignant serous effusions with a sensitivity of between 24% and 67%. The immunocytochemical staining for these markers resulted in malignant cells being detected in 18% to 33% of cases. Various combinations of conventional cytological examination, effusion fluid tumour marker determination, and immunocytochemical analysis identified malignant cells in serous effusions in up to 72% of cases; conventional cytology alone detected tumour cells in only 30%.