In order to validate xenografted small cell lung carcinomas (SCLC) for biological studies, the authors established 12 lung neuroendocrine (NE) tumors (eight typical SCLC and four atypical NE tumors [ANE]) by heterotransplantation onto nude mice. Their characterization was performed using serial ultrastructural, enzymatic, and immunohistochemical methods on primary tumors and after xenografts. These were subclassified into epithelial (one), neuroendocrine (three), and multidifferenciated (eight) types. The phenotypic characters (cytokeratins, neurofilaments, neurone-specific enolase) and the proliferative rate (Ki 67 labelling) of original tumor were maintained until the last passage studied. Although further acquisition of subsets of cytokeratin or neurofilaments was observed in some cases, the authors could not detect any morphologic and/or biological spontaneous change comparable to those described in in vitro cell lines. In addition, ANE are not quite identical to variant subclasses described in vitro. The authors conclude that the stability of heterotransplanted SCLC is an advantage in further biological studies.