The efficacy and toxicity of netilmicin, a new semisynthetic aminoglycoside, was clinically evaluated in fifty-two patients with moderate to severe infections with Gram-negative rods or Staph. aureus. Average duration of treatment was 14 days and mean total dose 2,960 mg. One-hour mean value of netilmicin serum concentration was 6.4 microgram/ml and mean trough value 1.2 microgram/ml. Forty-four patients were cured or improved. In twenty-one of them the effect could be attributed to netilmicin alone; the other twenty-three had a combined therapy. No improvement took place in five, but four of them could not be regarded as netilmicin failure. One patient with Pseudomonas aeruginosa infection was possible failure. The sensitivity of the causative bacteria to netilmicin was studied and compared with amikacin, gentamicin, sisomicin and tobramycin. Vestibular function and hearing acuity was thoroughly examined by electronystagmography and audiography. Drug-related VIIIth nerve damage could not be confirmed in any of our patients. Five patients showed a rise of serum creatinine of 30 mumol/l. This shows that netilmicin, similar to other aminoglycosides, is a potential nephrotoxic drug. Netilmicin appears to be an efficacious aminoglycoside and the oto- and nephrotoxicity is low, if careful attention is paid to the renal function and the serum concentrations of the drug.