Ninety-four patients receiving immunosuppressive therapy with azathioprine and prednisone after human cadaver kidney transplantation developed urinary tract infections and were treated with co-trimoxazole or another antibiotic in a controlled randomized prospective trial. The incidence of leucopenia in the group treated with co-trimoxazole (10.6%) was not significantly different from that in the group treated with other antibiotics (23.6%). Leucopenia when it occurred did so soon after transplantation at a time when the function of the renal transplant was poor in relation to the dosage of azathioprine given. In all cases the temporary withdrawal of azathioprine relieved the leucopenia despite continuation of the co-trimoxazole treatment. This study did not provide any evidence that co-trimoxazole plus azathioprine was a more potent cause of leucopenia than azathioprine alone.