In seeking a source of a pure population of a single type of endocrine cell, the splenic lobe of the chick pancreas was investigated. This was because, in adult life, this lobe contains an abundance of glucagon cells and only a few endocrine cells of other types. The presence of cells containing insulin, glucagon, somatostatin, and pancreatic polypeptide was sought by immunocytochemical means in splenic lobes of normal chicks of 6, 7, and 8 days incubation as well as in splenic lobes which had been removed from donors of the same ages and cultured in isolation from the rest of the pancreas as chorioallantoic grafts. Insulin and glucagon cells were present in all specimens. Somatostatin cells were found in all splenic lobes except one from a normal 6-day embryo. Avian pancreatic polypeptide (APP)-immunoreactive cells were detected in all grafts from donors of 6 or more days incubation and, in normal splenic lobes, were first found at 7 days of incubation. In all grafted and normal splenic lobes older than 7 days on original isolation, glucagon- and APP-immunoreactivities were occasionally present in the same cells. Thus, although the splenic lobe cannot be used as a pure endocrine cell source, this study has demonstrated APP immunoreactivity in the chick pancreas at an earlier stage than has previously been reported, and has also revealed "double staining" of some pancreatic endocrine cells with antisera raised to glucagon and APP.