Mikamycin, an antibiotic effective against gram-positive organisms including resistant staphylococci, exhibits a marked activity against experimental infections of staphylococci, hemolytic streptococci and pneumococci.^1,2,3) It has been applied for clinical uses by a number of investigators and successful results have been accumulated. However there remains a discrepancy between a high in vivo activity and a low level in blood and tissues demonstrated by antimicrobial activity.^1~4) For the purpose of investigating this problem, mikamycins A and B were labeled with tritium; and the blood concentration, tissue distribution, and renal and biliary excretion were observed. The results are presented in this paper.