One hundred ten patients were randomized to receive one of the following antibiotic combinations: aztreonam + clindamycin, tobramycin + clindamycin, or amikacin + mezlocillin for the treatment of lower respiratory tract infections (LRTI) caused by gram-negative bacilli. Of the 68 patients who received aztreonam + clindamycin, 60 were clinically evaluable and 50 were bacteriologically evaluable. Of the 60 clinically evaluable patients, 54 were cured and 5 were treatment failures or died during the study period. Of the 50 bacteriologically evaluable patients, 46 were cured and 3 failed to respond to therapy. Of the 26 clinically evaluable patients in the tobramycin + clindamycin group, 22 were cured and 4 either failed to respond or died during the study period. Of 18 bacteriologically evaluable patients in this group, 16 were cured and 2 failed to respond. In the amikacin + mezlocillin group, 14 of the 15 clinically and bacteriologically evaluable patients were cured, and 1 failed to respond. The most commonly isolated pathogens were Klebsiella pneumoniae, Escherichia coli, and Pseudomo-nasaeruginosa. The very few adverse drug reactions that were seen were transient and comparable in all three groups except for renal function parameters, which deteriorated in 6–8% of patients receiving the aminoglycoside combination. All three antibiotic combinations were similar in effectiveness and safety.