A previous randomized trial from this department demonstrated that there was significant reduction in morbidity and post-operative bacteriuria when men undergoing transurethral prostatectomy were given a prophylactic perioperative regimen of 48 h of cefotaxime compared with a no treatment control group. In this paper we report the results of a new randomized trial to determine whether a single dose of cefotaxime given with the induction of anaesthesia would be of equal efficacy to the previous 48 h regimen. We also investigated the ward bacterial flora for cefotaxime resistance before and during the trial. The 48 h regimen was significantly superior to the single dose in terms of the number of men who developed post operative complications and there was also a small but significant reduction in post-operative hospital stay. Forty-eight hours of prophylaxis covers the normal time of post-operative urethral catheterization and this may explain our findings. There was no evidence of emergence of cefotaxime resistant strains.