The immunosuppressive properties of niridazole, an antihelminthic drug, have been investigated in rats. When given orally in a dose of 50 mg/kg, it extended the median survival of cardiac allografts from 7 to 20 days. The immunosuppressive effect was not increased by giving either azathioprine or prednisolone concurrently but when all three drugs were combined, immunosuppression was profound and only two of eight grafts were rejected. Drug combinations incorporating niridazole at a lower dosage or for a shorter period were less effective, and azathioprine and prednisolone on their own or used together prolonged graft survival only marginally in this model. It is concluded that niridazole is a powerful immunosuppressive drug in this species and a synergistic effect can be obtained by using it in combination with azathioprine and prednisolone.