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. Subacute toxicity studies of MCR in rabbits were carried out by drip intravenous infusion (d.i.v.) comparing with intramuscular injection (i.m.) for 30 days (doses; d.i.v. = 4, 25, 63 mg/kg, i.m. = 63 mg/kg). The results of the studies are as follows: Animals did not die at any dose. Renal disorders occurred mainly at the dose level of 63 mg/kg; they were almost similar to those observed when administered by i.m., and the grade of disorders in d.i.v. was the same as in i.m. The maximum safety dose was 4 mg/kg.