Repeated-dose toxicity of hyrubicin ID6105, a novel anthrarycline anticancer agent, was investigated in Sprague-Dawley rats. ID6105 was injected intravenously to rats at dose levels of 0.04, 0.2 or 1.0 mg/kg/day for 4 week. As a result, there were no dose-related mortality and specific clinical signs of all animals treated with the drug. However body weight gain of both male and female rats treated with a high dose (l.0 mg/kg/day) of ID6105 significantly decreased compared to control. Interestingly, the numbers of RBC and platelets, and concentration of hemoglobin remarkably increased, while protein synthesis was suppressed, which may be related to the atrophy of spleen, thymus and liver. Moreover there were severe lymphocytic depletion in spleen and thymus as well as decrease in the number of hematopoietic cells in bone marrow. Also, degeneration of cardiac muscles and testicular germinal epithelia were observed. Taken together, it is suggested that Long-term administration of ID6105 at high doses over 0.2 mg/kg/day might cause hematopoietic and male reproductive system injuries, in addition to hepatic dysfunction.