Plasma C-peptide and serum insulin antibody levels were determined in 5 diabetic patients undergoing vascularized pancreatic transplantation. The grafts functioned well at first and exogenous insulin could be withdrawn, but one to 7 weeks later the grafts were rejected. After the transplantation there was an increase in the fasting plasma C-peptide level, and B-cell stimulation with glucose or glucagon evoked a C-peptide response. Healing of ischaemic damage was reflected in an increase in the C-peptide level. During graft rejection the C-peptide level fell. Measurement of plasma C-peptide levels provides a direct index of the B-cell function of the pancreatic graft. After transplantation the insulin antibody level fell exponentially, with an apparent half life of 10-11 days, whereas the level of total IgG was variable. The results indicate that formation of insulin antibodies ceases immediately on removal of the immunogenic stimulus, that is, on withdrawal of xenogeneic insulin.