Transplantation of whole pancreas or pancreatic islets remains a promising approach to treatment of diabetes mellitus. Because at present there is no efficient method for in vivo early diagnosis of pancreatic islet rejection or for disorders of pancreatic endocrine function, we examined if dithizone (DTZ) and a synthetic iodo-derivative of DTZ (I-DTZ) can be used as a potential radioactive marker for monitoring viable transplanted pancreatic islets. Human pancreatic islets harvested from multiorgan donors were tested ex vivo after intraductal injection of DTZ solution for islet staining. Lewis rats were used in the in vivo experiments for localizing pancreatic islets in situ after intravenous injection of various concentrations of DTZ or I-DTZ. Fresh rat islets were transplanted into streptozotocin-induced diabetic recipients, either underneath the kidney capsule or intraperitoneally. Intravenous DTZ or I-DTZ was then used for macroscopic and microscopic identification of viable transplanted islets. These studies indicate that DTZ and I-DTZ solutions specifically stain pancreatic islets in vivo after intravenous injection without damage to their endocrine function as assessed by plasma insulin and glucose levels. Human islets stain red in in vitro studies similar to the DTZ (I-DTZ) effect in rats. We conclude that DTZ and I-DTZ are effective in the in vivo and in vitro identification of pancreatic islets and may have potential clinical application in the detection of pancreatic islet tumors (insulinomas) and in the diagnosis of rejection of pancreatic organ allografts or islets.