Porcine and human pancreatic islets were microencapsulated in an alginate-polylysine biomembrane and put in a chamber of a new vascular prosthesis composed of an inner tubing of Dacron mesh and an outer tubing of expanded polytetrafluorethylene material. The vascular prosthesis was anastomized between the iliac artery and the contralateral vein of diabetic dogs. The recipients did not receive any immunosuppressive therapy. Function of porcine and human islets was monitored by measuring serum glucose levels and human C-peptide concentrations. After transplantation, serum glucose levels were maintained at values lower than 200 mg/dl, and C-peptide concentrations were between 0.8 and 3.2 ng/ml. Injected insulin requirements decreased by 50%-60%. Four to 8 weeks after transplantation, histologic examination showed well-preserved and functioning islets in the majority of intact microcapsules. Fibrin and inflammatory cells were not observed in the chamber. These data suggest long-term survival and function of microencapsulated pancreatic islets in the vascular prosthesis.