The effect of 15-Deoxyspergualin, a novel drug which has been described to have anti-tumour activity, on allogeneic graft survival (Dark Agouti----Lewis rats) after pancreatic islet transplantation was tested. A marked prolongation of graft survival could be shown using doses of 1.0, 2.5 and 5.0 mg Deoxyspergualin/kg on day 0 until day +9 post transplantation. A maximum of 55.6 days (average) survival time was observed using 2.5 mg/kg Deoxyspergualin compared to 5.2 +/- 0.6 days without immunosuppression. Using the chemiluminescence reaction of recipient monocytes after islet transplantation, a marked suppression of the monocyte system exceeding the treatment period could be observed. Since, in contrast to cyclosporin, B-cell toxicity could not be shown, the new drug seems to be a hopeful step towards successful allogeneic islet transplantation for treatment of diabetes.