Specimens of peritoneal fluid or peritoneal washings from a series of 106 patients who had had laparotomy or laparoscopy for gynaecological complaints were studied "blind" by conventional cytology and immunocytochemical staining. The antibodies used were Ca 1 or Ca 2, anti-CEA, and HMFG-2 or E29. All these are directed against epithelial antigens and are expressed on most malignant epithelial cells and weakly or not at all on mesothelial cells. It was hoped that these reactions would confirm diagnoses made by conventional cytology and possibly show malignant cells which had not already been identified. Of 28 patients with malignant disease (chosen to exclude any with frank ascites), eight gave positive immunochemical reactions, only four having been reported positive from conventional examination. Of 77 patients without malignant disease, HMFG-2 or E29 gave positive reactions in seven, Ca 1 or Ca 2 in two, and anti-CEA in two (reactions with plasma cells being disregarded). Some misleading reactions were probably due to endometrial cells. It was concluded that the antibodies used in this study are not sufficiently specific or sensitive to allow immunocytochemical staining to replace conventional cytological diagnosis but are a useful supplementary aid.