The effectiveness of cefazolin or cefotaxime as antibiotic therapy was compared with that of ceftriaxone in a multicentre prospective randomized trial involving 1,254 consecutive patients operated upon for abdominal diseases. Patients about to undergo surgery of the colon or who had localized or generalized peritonitis at the time of operation were excluded from the study. The patients entered were divided into 4 strata according to the degree of operative contamination and to risk factors. In each stratum, the patients were allocated at random to one or the other of 2 treatment groups. Group 1 patients received cefazolin or cefotaxime in 3 doses of 1 g administered 8-hourly, the first dose being injected during induction of general anaesthesia. Group 2 patients received one single 1 g dose of ceftriaxone injected during induction of anaesthesia. There was no significant difference between the two groups in the wound infection rate and in the frequency of post-operative intra-abdominal abscesses. Although the percentage of post-operative pulmonary and urinary tract infections was lower in the ceftriaxone group than in the cefazolin/cefotaxime group, no significant difference was observed between the two groups in the number of patients who required curative antibiotic therapy. This study shows that one single dose of ceftriaxone is as effective as three doses of cefazolin or cefotaxime in preventing would infections and post-operative intra-abdominal abscesses, and that it is more effective in preventing extra-abdominal infections complicating surgery.