IN 1955, Zollinger and Ellison1reported an association between islet cell tumors and recurrent, frequently atypically located peptic ulcers. Since that time numerous cases of the "ulcerogenic syndrome" have been reported. However, the nature of the islet cell responsible for this syndrome has remained in doubt. All islet cell lesions, including hyperplasia, neoformation, adenoma, and carcinoma, which case reports are in the files of the Ohio State University, and 36 neoplasms of the pancreatic islets, which case reports are in the files of the Armed Forces Institute of Pathology, have been reviewed. All cases from the Ohio State University were examined using the following techniques: hemotoxylin and eosin,2chrome alum hemotoxylin-phloxin,2Heidenhain-azan,2Hellerstrom and Hellman's silver technique for "alpha" cells,3the rhodocyan technique of Glenner and Lillie,4and aldehyde fuchsintrichrome.2 With these techniques, four islet-cell types can be identified: the alpha, beta,