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. Fertility studies of MCR in rats were carried out by intravenous injection for safety evaluation (Dose: 25, 50 mg/kg and 75 mg/kg). The results of studies are as follows. There was no adverse effect on fertility ability at any dose. Suppression of adult body weight gain was observed slightly at all dose level in male and at the dose level of 75 mg/kg in female.