Facing the limited availability of human adult and fetal pancreases, fetal pig proislets (pancreatic islet precursors) were investigated in view of several inherent advantages. Six litters of fetuses of mean ± SE gestational age 75 ± 3 days were obtained from commercially available farm pigs. Pancreatic tissue was gently digested with collagenase, then a 10-day culture was performed. During culture, fetal proislets showed no insulin response to glucose alone but a significant response to glucose plus theophylline. The insulin content per microgram of DNA in the cultured proislets continuously increased. Histological examination by immunoperoxidase staining showed that, apart from single insulin- and glucagon-positive cells, there were no discrete islets in the pancreatic tissue and the cultured proislets. Diabetes was induced with streptozocin (STZ) in eight nude mice 3-4 wk after proislet transplantation and in another eight nude mice without transplantation. During the initial week, blood glucose levels of mice in both groups increased rapidly. The mean ± SE peak value of blood glucose levels in the transplanted group was 20.4 ± 2.0 mM and was 20.1 ± 1.3 mM in the group without transplantation. Simultaneously, body weight decreased from 29.5 ± 0.7 to 21.5 ± 0.9 g and from 27.9 ± 0.7 to 19 ± 1 g in the groups, respectively. Afterward, blood glucose levels of mice in the transplanted group gradually decreased, and normoglycemia was achieved in all mice within 50 ± 13 days after injection of STZ, i.e., 74 ± 13 days after transplantation. The group without transplantation persistently maintained blood glucose levels >16.7 mM. An intraperitoneal glucose tolerance test in five transplanted diabetic mice with normalized blood glucose levels showed a flat response curve. Grafts from two diabetic mice that had reverted to normoglycemia for >3 wk showed a significant response to 16.7 mM glucose alone and to 16.7 mM glucose plus 10 mM theophylline during perifusion. Abundant ducts, β-cell nests, mature islets, and vascularization were found with immunoperoxidase staining in sections of grafts derived from nondiabetic nude mice 2 mo after transplantation and also from diabetic mice that became normoglycemic by transplantation. Our results show that the tissue-culture method effectively produces and purifies fetal pig proislets. The purified proislets develop and differentiate in vivo but not in vitro. This study demonstrates for the first time that STZ is not toxic to fetal pig pancreatic α- and β-cells and that, with this new procedure, fetal pig proislets persistently reverse diabetes in nude mice.