The tolerance and efficacy of netilmicin (NM) were studied in pediatric patients who had gram-negative bacillary pneumonia. 16 patients ranging in age from 19 months to 19 years were enrolled in the study. Sputum was obtained through percutaneous transtracheal aspiration (14 patients) or suctioning through endotracheal tube (2 patients) and was cultured for aerobic and anaerobic bacteria. More than one pathogen was isolated in 14 of the 16 patients, and anaerobic bacteria were recovered from 7 of them. The gram-negative bacilli recovered included Pseudomonas aeruginosa (9), Klebsiella pneumoniae (7), Serratia sp. (5) Escherichia coli (4), and Proteus mirabilis (1). The pretreated clinical isolates were tested for in vitro susceptibility. In vitro testing showed no single aminoglycoside to be clearly superior to the other. The median minimal inhibitory concentration of NM, gentamicin, tobramycin and amikacin ranged between 0.5 and 2 μg/ml. NM was administered i.m. at a mean daily dose of 5.1 mg/kg. The duration of therapy was 10–14 days (mean 11.6). The mean concentration of NM 1 h after the first dose was 3.3 μg/ml. When measured after the 2nd day of therapy, the peak was 3.8 μg/ml and the through 0.9 μg/ml. Concomitant antimicrobial agents were administered during therapy with NM in 12 patients, but only in 2 they were potentially effective against aerobic gram-negative bacteria. No local or systemic adverse reactions were noted. Among the 14 evaluated cases there was complete cure in 12 and partial cure in 2. NM was thus found to be safe and effective in the treatment of gram-negative pneumonia in chi