AbstractImmunohistochemical staining of formalin fixed paraffin sections with the mouse monoclonal antibodies E29 and CAM 5·2 effectively distinguish epithelium (antigen positive) from stroma (antigen negative) in normal endometrium. These antibodies were used as epithelial markers in the study of eight malignant mixed Müllerian tumours (MMT) of endometrium and gave normal reactions with well differentiated neoplastic glands; in contrast, negative or very weak staining was observed in poorly differentiated epithelial cells, present in large numbers in three cases. Abnormal antigen‐containing cells were observed in the stroma of seven MMT. In some cases these are probably due to stromal invasion by malignant epithelium but in others they may represent an early stage of epithelial differentiation in mesenchymal cells of the malignant str