DW-166HC (^(166)Holmium (^(166)Ho)-Chitosan complex) is a new radiopharmaceutic anticancer agent with a broad anti-tumorigenic spectrum, especially against human hepatic cancer. DW-166HC was evaluated for the appearance of micronucleus in polychromatic erythrocytes (PCEs) of mouse bone marrow cells after subcutaneous arid intravenous single administration. Bone marrow cells were prepared at 24 hr and 48 hr after DW-166HC-I (^(165)Ho-Chitosan complex : cold compound) administration and at 24 hr, 72 hr and 2 weeks after DW-166HC (^(166)Ho-Chitosan complex : hot compound) administration. The results showed there was no statistically significant increase of the numbers of PCEs with micronucleus in all DW-166HC-I administered groups compared with a negative control group but there was statistically significant increase of the numbers of PCEs with micronucleus at 24 hr arid 72 hr in all DW-166HC administered groups, which was recovered after 2 weeks from the drug administration. The results also showed the ratio of normochromatic erythrocytes (NCEs) to PCEs of all DW-166HC-I administered groups was not significantly different from that of a negative controi group but there was significant difference of this ratio at 24hr and 72 hr in all DW-166HC administered groups compared with that of negative group, which was also recovered after two weeks from the drug administration. These results suggested that DW-166HC-I may not cause any chromosomal damage but DW-166HC has in vivo mutagenic potential because of its radioactivity.