Fundamental and clinical studies on sisomicin, a new aminoglycoside antibiotic were carried out and the following results were obtained. 1. Antibacterial activity of sisomicin was superior to that of gentamicin, amikacin, cefazolin and ampicillin against S. aureus and Ps. aeruginosa. Against E. coli and K. pneumoniae, it was superior to that of amikacin, cefazolin, ampicillin and equal or slightly superior to that of gentamicin. 2. Mean serum levels of sisomicin were 18.0 +/- 0.5 microgram/ml, 15.7 +/- 1.3 microgram/ml, 9.6 +/- 0.6 microgram/ml, 3.2 +/- 0.4 microgram/ml, 1.7 +/- 0.2 microgram/ml and 0.95 microgram/ml at 1/2, 1, 2, 4, 6 and 8 hours after a single intramuscular administration of sisomicin 2.0 mg/kg to 3 children. Mean half-life time was 1.6 +/- 0.2 hours. Mean urinary recovery was 60.4 +/- 7.5% within 6 hours after administration of sisomicin 1.6-2.1 mg/kg to 4 children. 3. Sisomicin was given intramuscularly to 13 children with acute pyelitis (11), acute cystitis (1) and stomatitis gangrenosa and cervical lymphadenitis (1). The daily dose was 2.0 -4.2 mg/kg, divided into twice. Clinical response was excellent in 8 and good in 5. In bacteriological examinations, 13 pathogens (E. coli 9, K. pneumoniae 2, Ps. aeruginosa 2) were eradicated after administration. No adverse reactions were observed.