The in-vitro activity of tobramycin was compared with that of gentamicin in 133 isolates of Pseudomonas aeruginosa. All isolates were sensitive to tobramycin, whereas 11% were relatively resistant to gentamicin. In renal failure tobramycin can be prescribed effectively and safely in an intramuscular dosage of 1 mg/kg every 12 to 24 hours, the dose being varied according to blood assay results or serum creatinine levels. In recurrent Pseudomonas infections tobramycin has a place when gentamicinresistant strains have been selected.