Histologic, immunofluorescence and ultrastructural studies were performed in 17 cases of pancreatic carcinomas induced by the BK virus in Syrian hamsters, a unique model of experimentally induced malignant islet cell tumors. The tumors were composed of small, poorly differentiated cells mostly arranged in a trabecular structure. By immunofluorescence all four islet cell types were found in the tumors, though with different frequency. Insulin cells were present in 16 cases, glucagon cells in 11, somatostatin cells in 7, PP cells in 6. Thirteen tumors contained more than one cell type. Insulin cells were the most frequent cell type in 13 cases, and glucagon cells predominated in 1 case. Insulin-containing cells usually occupied a central position within tumor-cell aggregates, while the other cell types were mostly located in a peripheral position, a distribution reminiscent of that seen in normal islets. Gastrin and calcitonin immunoreactivities were not observed. Immunoreactive cells were more abundant in tumors with trabecular structure. Argyrophil cells revealed by the Grimelius method often exceeded the cumulative number of immunoreactive cells in the same tumor, which suggests that there were additional cell types. Multiple cell types were also found in liver metastases. Ultrastructurally most neoplastic cells were poorly granulated. The occurrence of many damaged cells suggests hormone leakage, which may account, at least in part, for the deregulated hormone release from the tumors.