Using a battery of monoclonal antibodies, we investigated the cytokeratin pattern in the epidermides of 12 human ovarian mature cystic teratomas (MCTs) and compared them with those of infant, adult, and fetal skin. Histologically, two types of epidermal layers were identified in the MCTs, a mature layer and an immature layer. The mature layer was similar to the epidermis of infants and adults, while the immature layer resembled stratified nonkeratinizing and metaplastic squamous epithelium. The cytokeratin pattern of the histologically mature epidermis in MCT was either similar to that in infants and adults or was of the fetal type. The cytokeratin expression of the histologically immature epidermis in MCT also showed many similarities to the fetal cytokeratin pattern. We conclude that histologic maturity of the epidermis in MCT is not necessarily expressed by the cytokeratin pattern, which reflects the state of molecular rather than histologic differentiation. Since prognosis in germ cell tumors is usually related to the degree of tissue maturation, our observations raise the possibility that the cytokeratin profile may eventually prove to be a valuable prognostic tool in some of the neoplasms that contain epithelial elements.