Bacterial cultures of graft perfusion fluid were taken on 514 renal perfusions during a ten-year period. A total of 22 positive cultures were found. Two sets of kidneys were discarded because of the bacteriology report. Only two possible wound infections were found that could be attributable to the preoperative contamination even though 19 of 24 patients with "contaminated" kidneys received no antibiotics. No loss of kidneys or life was found. Of the 22 contaminated perfusates, 18 (80%) originated from other institutions although only 144 (28%) of the total kidneys that were perfused came from other institutions. The majority of cultures indicated that the organisms were skin contaminants or organisms of low virulence. Microbial contamination of perfused cadaver kidneys is uncommon (1.2% in local kidneys and 9.3% in shared kidneys). Contaminated kidneys are not a major source of infection in renal transplant recipients. Kidneys that are contaminated with frequently virulent Gram-negative bacteria, such as Escherichia coli, should be discarded. Contamination of kidneys with Gram-positive organisms should not be a contraindication for transplantation, but the patients should probably be given prophylactic antibiotics.