Small cell carcinoma of the pancreas and biliary tract. Four cases of anaplastic carcinoma of the pancreas or biliary tract were studied clinicopathologically and immunohistochemically. All four cases were intermediate cell type and contained a minimum amount of microscopic foci of differentiated glandular adenocarcinoma. Argyrophilic tumor cells were not seen in any of the four tumors. Immunohistochemically, no tumor was positive for hormonal products, but all tumors were positive for epithelial markers. These findings suggest that the anaplastic carcinoma are not derived from argyrophilic cells, but rather from adenocarcinomas which have the potential for anaplastic metaplasia. The long-term survival of one patient emphasized the importance of chemotherapy in the treatment of small cell carcinoma of the pancreas and biliary tract. 