Using an indirect immunoperoxidase staining technique we have demonstrated the plasma protein, prealbumin, in the cells of the pancreatic islets of Langerhans. Serial sections stained with antisera to glucagon, insulin and somatostatin revealed that the distribution of the prealbumin stained cells matched that of the glucagon A-cells. Absorption and cross-absorption experiments with prealbumin and glucagon showed that the staining was specific. We also found positive staining for prealbumin in single cells in the mucosa of the gastrointestinal tract, mainly stomach and colon, and kidney tubules.