An antiserum raised against human milk fat globule membranes has been used to stain a wide variety of human tissues by the indirect immunoperoxidase method. The antigen detected has been designated Epithelial Membrane Antigen and was confined to the luminal and surface membranes (and to a lesser extent the cytoplasm) of epithelial tissues in the normal state. Increased staining was observed in a variety of nonneoplastic disease states as well as in many neoplasms. The staining of tumors is related both to their histogenesis and degree of differentiation, being found only in lesions of surface epithelial or mesothelial origin and being more consistently present in well and moderately differentiated neoplasms. However, strong staining was also present in many anaplastic tumors and this together with the resistance of the antigen to formol fixation and paraffin embedding suggest that the antiserum has a valuable role to play in diagnostic tumor histopathology as an indicator of epithelial differentiation. The distinction of anaplastic carcinomas from malignant lymphomas and the recognition of spindle-cell epithelial malignancies are especially useful applications. The identification of minute metastatic deposits of carcinoma in organs such as the liver and bone marrow is also greatly facilitated. Malignant neoplasms often exhibit patterns of staining different from those of the normal tissues. The possible significance of these patterns is discussed.