In order to assess the practical value of immunocytochemical techniques in routine cytological diagnosis, a panel of three monoclonal antibodies (anti-CEA, Ca 1 and HMFG-2) were used to search for malignant cells in 53 samples of pleural or peritoneal fluid from 41 selected patients. All these patients suffered from malignant disease, but neoplastic cells had not been reported on conventional cytological examination. In 12 of the 41 cases immunocytochemical staining revealed previously unrecognised malignant cells. This represents an increase in diagnostic accuracy of approximately 20% and suggests that immunocytochemical labelling should be used routinely for the examination of cytologically negative samples from patients with suspected malignant disease.