Indium Chloride-induced Micronuclei inIn Vivo and In Vitro Experimental Systems: Ryo Takagi, et al. Department of Public Health and Environmental Medicine, Jikei University School of Medicine-Objectives: The aim of this study was to investigate the genotoxic effects of indium trichloride (InCl 3 ·4H 2 O; InCl 3 ) using the in vivo bone marrow micronucleus test and the in vitro CHL/IU cell micronucleus test. Method: BALB/c mice were administered a single intraperitoneal (i.p.) injection of InCl 3 at a dose 0.625, 1.25, 2.5, 5, or 10 mg/kg b.w. The frequency of micronuclei, the ratio of polychromatic erythrocytes to normochromatic erythrocytes (P/N ratio) and body weight gain were determined 24 h after administration of the InCl 3. In the in vitro micronucleus test, CHL/IU cells were treated continuously for 24, 48, or 72 h in the absence of S9mix (the continuous treatment method) and/or for 6 h with or without S9 mix followed by an 18, 42 or 66 h recovery time (the short time treatment method). The frequency of micronuclei was determined at the end of each culture period. Results: The frequency of micronuclei induced by InCl 3 increased in the in vivo erythroblast-erythrocyte micronucleus test using BALB/c mice at doses of 2.5 and 5 mg/kg b.w. The P/N ratio, a marker of bone marrow toxicity, decreased significantly following the injection of InCl 3 . Body weight gain was also inhibited by InCl 3 . InCl 3 induced micronuclei in the CHL/IU cell micronucleus test in both the continuous treatment method and the short time treatment method, both with and without S9mix. Conclusions: These results suggest that InCl 3 has a genotoxic effect on mammalian cells both in vivo and in vitro. (J Occup Health 2011; 53: 102-109)