This report describes the results of extensive laboratory and clinical evaluation of gentamicin in infants and children. Of more than 4,000 strains of gram-negative bacteria, over 95% were susceptible to the antibiotic in vitro. The results of studies of the absorption, excretion, and distribution of gentamicin in different doses in subjects ranging in age from the neonatal period through adolescence are described. Sixty-nine of 76 patients with bacterial gastroenteritis responded to the drug administered orally. The therapeutic response was considered satisfactory in approximately 75% of patients with serious infections, including pyelonephritis, pneumonia, meningitis, septicemia, and others due to gram-negative bacteria. There was no evidence of serious toxicity due to gentamicin. The preliminary pharmacologic results with an experimental formulation containing 10 mg of gentamicin, intended for use in neonates and young infants, were encouraging.