Sixty-six patients with acute intraabdominal infections due to gram-negative aerobic organisms were treated with aztreonam plus clindamycin or with tobramycin plus clindamycin in a multicenter, comparative, randomized study. The patients had undergone a variety of surgical procedures; most of them had peritonitis. Thirty-three of the 36 patients in the aztreonam group and 26 of the 30 patients in the tobramycin group had satisfactory clinical responses. Only one gram-negative aerobic pathogen, a strain of Pseudomonas aeruginosa, persisted after treatment; the patient involved was in the tobramycin group. The incidences of adverse reactions, superinfections, and abnormal laboratory values were low in each treatment group. The difference between the efficacies of the two regimens was not statistically significant. This study suggests that aztreonam may be a useful alternative to the aminoglycosides in the treatment of gram-negative intraabdominal infections.