The administration of antibiotics to renal transplant patients using cyclosporin can be difficult because of the risk of severe nephrotoxicity. An investigation was therefore carried out to determine whether aztreonam, a synthetic monocyclic beta-lactam antibiotic, can safely be combined with cyclosporin. In this retrospective study 68 renal transplant patients who received preoperative antibiotic prophylaxis consisting of aztreonam, ampicillin, and lincomycin were compared with 68 patients who received ceftazidime instead of aztreonam. Both groups were treated with cyclosporin and prednisolone and followed for 3 months. After transplantation 28.7% of the patients suffered from an acute renal failure and 1.5% had a wound infection. There were no significant differences between the two groups in acute renal failure, would infections, other infections, incidence of rejections, duration of admission, or graft survival. We therefore conclude that aztreonam can safely be administered together with cyclosporin. We also conclude that the combination of aztreonam, ampicillin, and lincomycin is a good preoperative antibiotic prophylaxis in renal transplant patients.