A specific and saturable interaction between 125I-gastrin and eosinophils was discovered in autoradiographs of human gastric mucosal tissue and confirmed in isolated and enriched preparations of WBC's. Gastrin displaced 125I-gastrin from eosinophils in a dose-dependent manner with a D50 = 11 uM. Scatchard analysis of the saturation curve indicated a single binding site of low affinity (Kd = 4.14 uM) and high capacity (Bmax = 430 umoles/mg protein). The gastrin binding protein was localized to the granular core of the eosinophil and found to have a molecular weight of approximately 15 kDa following chemical crosslinking of radioligand to granules and SDS/PAGE. Based on its molecular weight and granular location and the charge characteristics of gastrin, the gastrin binding protein in the human eosinophil is most likely major basic protein. In vivo this interaction might act to limit the cytotoxic potential of MBP on tissues and/or attentuate gastrin concentrations thereby helping regulate gastric acid secretion and mucosal growth.