Mutation frequency tests for isepamicin (HAPA-B), a new aminoglycoside antibiotic, were carried out using Salmonella typhimurium strains TA98, TA100, TA1535 and Escherichia coli WP2uvrA. After a 30-minute exposure of bacterial cells at 37 degrees C to HAPA-B in a buffer solution, the cells were washed twice with a buffer solution to minimize the cytotoxicity of the drug. Then washed cells were inoculated on minimum glucose agar plates to determine mutation frequency. The results should no significant increase in mutation frequency with increasing HAPA-B concentration; suggesting that the drug has no mutagenicity detectable by this test system.