The product of c-erbB-2 gene is detected in a proportion of carcinomas from various sites and is generally associated with a high degree of malignancy. A series of 58 effusions containing malignant cells and 16 cytologically negative serous effusions was assessed by immunocytochemical methods for c-erbB-2 expression using the monoclonal antibody NCL-B11, which recognizes the internal domain of the c-erbB-2 oncoprotein. Both alcohol-fixed smears and cell blocks from formalin-fixed specimens were used. A crisp, clear cut membrane-associated positive staining was evident in 51% (30/58) malignant effusions and was restricted to metastatic adenocarcinomas. Breast and ovarian cancers showed the highest incidence of positivity. Mesotheliomas as well as non-neoplastic effusions were consistently negative. Paraffin blocks from formalin-fixed cells displayed a weak immunoreactivity when compared with their alcohol-fixed counterparts. The study shows that the c-erbB-2 oncoprotein can be easily identified in standard cytological smears: it can be of assistance in differentiating adenocarcinomas from mesotheliomas, and in selected cases it can provide a further prognostic indicator, replacing tissue immunohistochemistry.