Tobramycin, a new aminoglycoside antibiotic, was evaluated in the treatment of 40 adult patients with systemic bacterial infections. In only 11 patients was there no serious underlying disease. Complete cure was obtained in 24 patients. In 11 patients, treatment was either partially effective or could not be properly assessed, and in 5 patients it was judged ineffective. The antibiotic was well tolerated and no serious adverse reactions were observed. Results of in vitro susceptibility tests demonstrated lack of complete cross-resistance between gentamicin and tobramycin and superior activity of tobramycin against P. aeruginosa isolates. Adequate peak serum tobramycin concentrations were achieved following 80 mg intramuscular injections but the levels fell below 1 μ/ml after 8 h, indicating that a 6-hourly dosage regimen may be necessary in patients with severe infections.