Mouse monoclonal antibodies have been used to study the distribution of the epidermal growth factor receptor in human cultured cells and tissues. As expected, most epithelial cells expressed epidermal growth factor receptor (EGFr), whereas cells of hematopoietic origin were EGFr-. However, EGFr was found to be differentially expressed on cultured cells of neuroectodermal origin. Normal melanocytes and a proportion of melanomas are EGFr-, whereas a distinct subset of other melanomas, astrocytomas, and neuroblastomas is EGFr+. Expression of the EGFr in melanomas was closely related to the expression of phenotypic traits of differentiation. Less differentiated melanomas have an epithelioid morphology and are nonpigmented, Ia+, and EGFr+; in contrast, more differentiated melanomas have a dendritic morphology and are pigmented, Ia-, and EGFr-. In the melanoma cell panel used, expression of EGFr did not correlate with rate of proliferation. Expression of EGFr in tissues also showed a lack of correlation with the proliferative state of cells. Our findings indicate that expression of EGFr is related to cell lineage and specific stages of cellular differentiation, rather than only to cell proliferation. The data suggest that receptor content may be elevated in a large number of tumor cell lines.