As methods for human islet isolation improve and clinical trials become widespread, some form of storage will be needed. Cryopreservation would offer many advantages: storage in a tissue bank to provide sufficient islets to transplant to individual recipients; modulation of tissue immunogenicity; purification; and, facilitate shipment from center to center. Since 1976, much work has been done on islet cryopreservation with cooling rates of 0.025 degrees C/min to 75 degrees C/min being reported as optimum. In rats we have found that slow cooling to -40 degrees C with rapid thawing from -196 degrees C gives the highest survival as measured in-vitro and in-vivo. This freeze-thaw protocol also provides viable pancreatic microfragments and pure dog islets which can both induce prolonged normoglycemia. This freezing procedure also gives prolonged survival of islet xenografts and purifies the pancreatic microfragments of the unwanted exocrine contaminants. This method of freezing is now being used for human islets. Low temperature storage of human islets will greatly help the clinical use of islet transplantation.