Pancreatic islet grafts transplanted into patients with autoimmune diabetes are potentially threatened by two immune responses, allograft rejection and the recurrence of autoimmune insulitis. In the present study we investigated the humoral autoimmune response directed to islet autoantigens by studying islet cell antibodies and glutamic acid decarboxylase (GAD 65) antibodies in twenty-one insulin-dependent diabetes-mellitus (IDDM) patients undergoing intraportal islet allotransplantation. Islet transplantation was performed according to the following recipient categories: Islet after kidney transplantation (n=10), simultaneous islet and kidney transplantation (n=6) and islet transplant alone (n=5). GAD 65 antibodies were detected in a radioligand GAD 65 antibody assay using recombinant, in vitro translated, human 35S-methionin labelled GAD 65 as tracer. Islet cell antibodies were determined by indirect immunofluorescence technique on human pancreas. In six out of twenty-one patients we observed GAD 65 antibodies before islet transplantation and the GAD 65 antibodies persisted despite immunosuppression. In contrast only two subjects were concordantly islet cell antibody positive and the titre decreased post transplantation. In addition we observed occurrence of GAD 65 antibodies in five subjects that were shown to be antibody negative before islet transplantation with three of them subsequently becoming positive for islet cell antibodies. The remaining ten patients were GAD 65 antibody and islet cell antibody negative before islet transplantation and remained negative thereafter. Interestingly none of the patients was exclusively positive for islet cell antibodies without being positive for GAD 65 antibodies. In summary we have demonstrated in twenty-one islet grafted individuals that humoral autoimmunity to islet antigens can persist or occur despite immunosuppression. Islet cell antibodies appear to be less frequent (5 out of 21, 23%) compared to GAD 65 antibodies (11 out of 21, 52%) suggesting that they are more affected by immunosuppressive therapy. We conclude that GAD 65 antibodies are a useful tool to further evaluate a possible link between persistent autoimmunity and early or late graft failure after islet transplantation.