The mutagenicity of cefodizime sodium (THR-221) was investigated by the reverse mutation test using seven bacterial strains (Salmonella typhimurium strains TA100, TA98, TA1535, TA1537 and TA1538, and Escherichia coli strains WP2 and WP2uvrA) and the chromosomal aberration test with cultured Chinese hamster lung (CHL) cells. The reverse mutation test was carried out in dose range from 0.0025 to 5.0 micrograms/plate in the absence and presence of mammalian metabolic activation system. Antibacterial effects were observed at concentrations more than 0.25 microgram/plate, and no significant increases in the number of revertants were observed at the dose levels where antibacterial effects were not detected. THR-221 caused no increases in the number of chromosomal aberrants at dose levels of 0.75, 1.5, 3.0 and 6.0 mg/ml in the absence and presence of the metabolic activation. These results indicate that THR-221 has no mutagenic activity.