In serous effusions, the cytologic diagnosis of adenocarcinoma, and its distinction from mesothelial hyperplasia and malignant mesothelioma, is still difficult. This study compares the immunoreactivity of antibody B72.3 with that of anti-CEA, anti-LeuM1-antigen and anti-keratins in 41 serous effusions from patients with different malignant disorders. B72.3 selectively stained adenocarcinoma cells in 80% of the cases and was as good as anti-CEA and superior to anti-LeuM1-antigen and anti-keratins. It is concluded that B72.3 forms a useful supplement to conventional cytomorphology in the distinction between adenocarcinoma cells, reactive mesothelial cells and malignant mesothelial cells in serous effusions.