Of the 42 Merkel cell carcinomas that we studied, two showed numerous tubular structures within sheets and nests of small cells. The small cells stained for both neuron-specific enolase and keratin. The keratin decorated a dot-like paranuclear structure. The ducts stained positively for carcinoembryonic antigen (CEA) and CF-1 (cystic fibrosis-1, a monoclonal antibody that only stains eccrine duct and acrosyringium). Electron microscopy performed on one case showed cytoplasmic dense-core neurosecretory granules and intercellular lumina lined by cells containing microvilli. These ultrastructural and immunohistochemical features support the concept of eccrine differentiation in these tumors. A third case contained foci of typical keratinizing squamous cell carcinoma admixed with sheets of small cells. The immunohistochemical and ultrastructural characteristics of this tumor were essentially similar to those of a conventional Merkel cell carcinoma. Our findings suggest that Merkel cell carcinomas, similar to neuroendocrine tumors from other anatomic sites arise from a primitive totipotential stem cell that has the capacity to differentiate along different cell lines.