The usefulness of a new commercially available monoclonal antibody (MoAb) BER-EP4 was evaluated. This MoAb is directed against a cell surface glycoprotein reported to be present in most epithelial cells and epithelial tumors but not in mesothelial cells. Cell block sections from 103 adenocarcinomatous and 129 benign effusions were studied. Positive staining was seen in 85 of the 103 (83%) malignant effusions. Immunoreactivity was seen in 73-93% of adenocarcinomas from the ovary, gastrointestinal tract, lung, and breast. Mesothelial cells from 3 of the 129 (2%) benign effusions showed positive staining. It is concluded that immunocytochemical staining with MoAb BER-EP4 is a sensitive and specific aid in distinguishing reactive mesothelial cells from adenocarcinoma cells in body fluids.