We used an intracellular zinc-specific fluorophore, Zinquin, in conjunction with fluorescence video image analysis, to reveal labile zinc in pancreatic islet cells, which concentrate this metal for use in synthesis, storage, and secretion of insulin. Zinquin vividly demonstrated zinc in the islet cell secretory granules, which formed a brightly labeled crescent in the cytoplasm between one side of the nucleus and the plasma membrane. Lower but still appreciable amounts of zinc were detected in the remaining cytoplasm, but there was little labeling in the nucleus. Fluorescence intensity varied among islet cells, suggesting differences in zinc content. Their average fluorescence intensity greatly surpassed that of the surrounding pancreatic acinar cells in frozen sections of pancreas and in all other types of cell studied, including lymphocytes, neutrophils, fibroblasts, and erythrocytes. Less labile zinc was detected in cells of the mouse insulinoma cell line NIT-1, regardless of whether they were maintained in long-term culture in the presence or absence of exogenous extracellular zinc. Exposure of islet or insulinoma cells to a high concentration of glucose or other secretagogue decreased the content of labile zinc. Zinquin should be a useful probe for revealing changes in zinc homeostasis in islet B-cells that may be important in their dysfunction and death during diabetes.