Single-shot antibiotic prophylaxis is well established in abdominal surgery. There is evidence suggesting that it prevents wound infections and some authors report also prevention against postoperative urinary tract infection and pneumonia. From April 1988 to December 1990 we randomly assigned 429 patients with gastro-intestinal operations to a defined protocol: 210 patients (5 drop-outs) with elective operations of the upper GI-tract were given Ceftriaxone (half-life 8 hours, 102 patients) or Cefazolin (half-life 2 hours, 103 patients). 117 (12 drop-outs) patients with operations of the lower GI-tract were given Ceftriaxone/Ornidazole (half-life 13 hours, 50 patients) or Cefazolin/Metronidazole (half-life 8 hours, 55 patients). 102 (20 drop-outs) patients with appendicitis were given Ornidazole (40 patients) or Clindamycin (42 patients). There were no differences in sex, age or type of operation in the different groups. The overall postoperative infection-rate was low. In the upper GI-tract we found one wound infection in both groups, in the lower GI-tract two wound infections in the Ceftriaxone/Ornidazole-group vs. nine in the Cefazolin/Metronidazole-group (p < 0.05). In patients with appendicitis there were three infections in the Ornidazole-group and four in the Clindamycin-group. There was no statistically significant difference in pulmonary or urinary tract infections in all groups. Although the protocol for antibiotics with a short half-life included a second dose of antibiotics in cases of operations with a duration of more than four hours, this was forgotten in 19 of 39 concerned patients (49%!).(ABSTRACT TRUNCATED AT 250 WORDS)