AIMS--To investigate the presence of the macrophage associated antigen CD68 in non-haematopoietic tumours. METHODS--Cytological specimens from non-macrophage derived tumours were stained using the alkaline phosphatase anti-alkaline phosphatase immunocytochemical method (APAAP) and three monoclonal anti-CD68 antibodies, Y1/82A, EBM11, and KP1. RESULTS--Reactivity of malignant cells with one or more of the antibodies was seen in 11 out of 40 adenocarcinomas and in one of seven poorly differentiated carcinomas; other neoplasms, including 10 cases of squamous carcinoma, three of malignant melanoma, and four of oat cell carcinoma were negative. Monoclonal antibody KP1 gave the strongest staining and reacted with the highest proportion of neoplastic cells. CONCLUSIONS--CD68 is expressed in a proportion of epithelial tumours although the labelling is usually less intense than in macrophages. Anti-CD68 antibodies should therefore be used as part of a panel in the diagnosis of poorly differentiated neoplasms in cytological material.