Porcine fetal endocrine pancreatic tissue was placed under the kidney capsule in a diabetic renal transplant patient. In a core-needle kidney biopsy specimen obtained 3 weeks after transplantation, clusters of epithelial cells were identified in the subcapsular space. The ultrastructural and immunocytochemical features of these cells were typical of pancreatic islet cells. Some cells stained positively for insulin; others stained positively for glucagon, somatostatin or chromagranin A. There were well-defined cytoplastic storage and transport granulae that indicated hormone synthesis. The ultrastructural findings provide further evidence that porcine cells can survive after transplantation to humans.