Acetyl-kidamycin administered intravenously to mice disappears rapidly from blood and is distributed widely in various organs, where much of it is adsorbed and/or inactivated. A large quantity of the antibiotic is found in the intestinal content indicating the possibility of excretion into the bile. Inactivation of acetyl-kidamycin with homogenates of mouse organs such as liver, intestine, kidney, etc. was observed in vitro. Tumor tissue shows low concentrations of acetyl-kidamycin but also low rates of inactivation. The inhibitory effects of acetyl-kidamycin on incorporation of ³H-thymidine were correlated with organ levels of the drug, but in the case of S-180 tumor, a marked inhibitory effect was observed even with a low concentration of the antibiotic.