The GB-04-JCK xenograft line of human gallbladder small cell carcinoma was established in nude mice by serial transplantation. The xenotransplantability has been maintained for more than 20 years. The carcinoma cells grew in a solid-sheet pattern and were found to have hyperchromatic nuclei, finely dispersed chromatin and inconspicuous nucleoli in the primary gallbladder tumor, as well as in the established xenograft GB-04-JCK The carcinoma cells also had Grimelius argyrophil granules, electron-dense neuroendocrine granules bounded by a single membrane. The xenograft line retained histological and immunohistochemical characteristics of the primary gallbladder tumor and is the first reported xenotransplantable tumor of human gallbladder small cell carcinoma.