The cytoskeletons of various human neuroendocrine (NE) tumors were analyzed immunohistochemically using antibodies against intermediate-filament (IF) proteins as well as by two-dimensional gel electrophoresis of proteins from microdissected tissue samples. All of the tumors studied were found to contain cytokeratin filaments and are therefore referred to as 'NE tumors of the epithelial type'. In addition, neurofilaments were found in most cutaneous and some pulmonary NE tumors, as well as in medullary carcinomas of the thyroid and in pancreatic islet cell tumors. The neurofilament staining was frequently concentrated in cytoplasmic IF aggregates. Gel-electrophoretic analyses showed that all NE tumors examined synthesize 'simple epithelium-type' cytokeratin polypeptides, cytokeratins nos. 8 and 18 being the most prominent ones, whereas cytokeratin no. 19 was found in variable and usually minor amounts. A new cytoskeletal protein, designated IT protein, with a relative molecular weight of 46,000 and an isoelectric pH value of approximately 6.1 (in 9.5 M urea) was detected in all 9 cases of cutaneous NE tumors ('Merkel-cell carcinomas'), including 2 lymph-node metastases, but was not found in any of the 17 cases of pulmonary NE tumors. In addition, 2 medullary carcinomas of the thyroid, 2 islet cell tumors of the pancreas, and 1 intestinal carcinoid tumor also seemed to lack this protein. A protein indistinguishable from IT protein by electrophoresis and tryptic peptide mapping was found in cytoskeletal preparations of mucosal cells of human intestine and in cultured human colon carcinoma cells of line HT-29. A possible relationship between IT protein and the type-I subfamily of cytokeratin polypeptides is discussed. Our study shows that the co-expression of cytokeratin filaments and neurofilaments may provide a criterion which is useful for the recognition of some NE tumors but which does not distinguish between NE tumors of different types and origins. In contrast, IT protein seems to be present specifically in cutaneous NE tumors, but absent in pulmonary NE tumors. The implications of these findings for the elucidation of the histogenesis of cutaneous NE tumors and for the histopathological differential diagnosis of NE tumors of cutaneous and pulmonary origin are discussed.