A neuroendocrine skin carcinoma cell line MKL-1 has been established from a nodal metastasis in a 26-year-old patient. The line grows as irregularly outlined, loosely packed floating aggregates lacking central necrosis. MKL-1 is hyperdiploid and has a mean doubling time of 120 hours. Xenografts of 2 X 10(7) MKL-1 cells produce tumors in nude mice at 4 to 6 weeks after subcutaneous inoculation. The xenografts were morphologically indistinguishable from the original skin primary and the nodal metastasis. Electron microscopy revealed sparse membrane-bound neurosecretory granules, and conspicuous, paranuclear aggregates of intermediate filaments. Immunohistochemical study showed diffuse and consistent staining with neuron-specific enolase, while bombesin, adrenocorticotrophic hormone, Leu-enkephalin, substance P, and vasoactive intestinal polypeptide displayed heterogeneous and variable expression. Uniform staining of all cells appearing as cytoplasmic fibrils and paranuclear aggregates was noted with antibodies to cytokeratin. Appreciable amounts of cytokeratin polypeptides 8, 18, and 19 and IT protein were seen on two-dimensional gel electrophoresis of cytoskeletal preparations from MKL-1 cells and from tumor-rich frozen sections. Immunostaining also showed coexpression of neurofilaments arranged in paranuclear aggregates; gel electrophoresis and immunoblotting demonstrated the presence in MKL-1 cells of prominent amounts of the small neurofilament polypeptide. Focal expression of desmoplakin was noted in the xenografts. The cells reacted with monoclonal antibodies anti-Leu-7 and anti-Leu-M1 but did not react with antibodies to human lymphocyte antigens (HLA)-A, HLA-B, and HLA-C. Cytogenetic analysis revealed the presence of 3 chromosomally abnormal cell lines with the majority of metaphase cells demonstrating a gain of an isochromosome of the short arm of chromosome 5. Thus, MKL-1 cell line shares several characteristics with small cell neuroendocrine bronchopulmonary carcinoma cell lines but shows distinct cytogenetic abnormalities.