Tissue distribution, excretion and metabolism of prumycin in normal mice and rats were studied by microbiological assay. Following the injection of prumycin into mice, high activity was detected and continued for 24 hours in the kidney, and the activity was also high in the skin, uterus, bone, liver, lung and stomach in this order. But concentration in the brain, heart, spleen and testis were too low to detect even 5 minutes after the injection. Prumycin was not inactivated by a variety of tissue homogenates in vitro. Therefore, inability to detect activity of prumycin in the spleen and testis appears to result from poor distribution rather than inactivation by these organs. About 70% of injected prumycin was excreted into rat urine in 24 hours but it was not detectable in feces. When prumycin was injected intravenously into dogs at the dose over 10 mg/kg, vomiting was observed in all animals, and LD₅₀ was about 50 mg/kg.