The identity and antibiotic sensitivity of the isolated bacteria from 179 patients undergoing transurethral resection of the prostate were analysed. The patients were randomized into a group receiving a short course of cefotaxime in conjunction with the operation and a control group given no antibiotics. Preoperatively 70 patients had bacteriuria (greater than or equal to 10(7) CFU/l) with a predominance of Gram-negative bacteria (57 isolates, mainly Escherichia coli, Enterobacter, Klebsiella ssp, Proteus ssp), although Gram-positive species (24 isolates, mainly enterococci and Staphylococcus epidermidis) were also frequently encountered. Preoperatively isolated pathogens were evenly distributed in both groups and the sensitivity pattern was comparable. In the cefotaxime group postoperative recurrence of the preoperatively identified bacteria occurred in a lower frequency (11/43) than persistence in the control group (24/38). Only Gram-negative pathogens were isolated from patients with postoperative septicemia and upper urinary tract infections indicating that it is most important to direct prophylaxis against Gram-negative bacteria. A high degree of sensitivity against cefotaxime, gentamicin, trimethoprim, co-trimazine and a combination of ampicillin and mecillinam was recorded among both pre- and postoperatively isolated bacteria.