By use of an assay system in vitro with cultured FM3A cells from a C3H mouse mammary carcinoma, tetracycline hydrochloride was found to have the ability to induce an 8-azaguanine-resistant mutation. It is suggested that an assay system using mammalian cells for the detection of mutagenicity of certain substances having antimicrobial action is more practical than a system using bacteria. As other effects of tetracycline hydrochloride on FM3A cells, the damage on chromosomes and the inhibition of syntheses of protein and nucleic acids were demonstrated.