Zinc iodide-osmium tetroxide (ZIO) fixation/staining technique is a metallophilic technique which has been used for the examination of various tissues and cell types. We examined the ZIO (+) cell types in rat exocrine pancreas to obtain further evidence for the significance of the reaction. Among mostly non-reactive pancreatic acinar cells there were ZIO (+) acinar cells of varying staining intensity. Zymogenic granules and centroacinar cells were completely non-reactive. Our electron microscopic findings support the view that the reactivity of the technique used is cell specific but not cell type or organelle specific.