Micronomicin (MCR) is a new aminoglycoside antibiotic produced by Micromonospora sagamiensis var. nonreducans which was isolated from soil collected at Sagamihara City by Nara et al. This antibiotic shows a close similarity to gentamicin C components in physical and chemical properties. The antibacterial activity of MCR is broad-spectrum and almost equal to that of gentamicin C complex. MCR exhibits particularly high activity against Pseudomonas, Proteus, Klebsiella pneumoniae, Serratia, etc. as well as against some Pseudomonas aeruginosa strains resistant to gentamicin C1a. Toxicological studies of MCR in rats were carried out by intravenous injection for safety evaluation. Study on subacute toxicity: Wistar rats were injected intravenously with MCR at the dose levels of 4, 10, 25, 63 mg/kg and 100 mg/kg for 30 days. The results of the studies are as follows: In the subacute toxicity study, animals died at the dose level of 100 mg/kg (10 out of 30 animals). Main changes observed were renal disorders and ataxia which showed a close similarity to those seen in intramuscular toxicity studies in rats. The renal histological disorders occurred mainly at the dose levels of 25 mg/kg and over, but they were slight at the dose levels of 25 mg/kg. Ataxia was observed at the dose levels of 63 mg/kg and over, but its grade was slight at the dose level of 63 mg/kg. The maximum safety dose was equal to in the intramuscular subacute toxicity in rats, 10 mg/kg.