The aim of this study was to investigate the in vivo effects of tetra (tetralet) antibiotic on chromosomal aberrations (CA) in bone marrow cells of rats (Rattus norvegicus var. albinos). The tetra antibiotic significantly increased the percentage of abnormal cells and the chromosomal aberrations per cell (CA/cell) in bone marrow cells of rats at concentrations of 100 and 200 mg/kg body weight for 12 and 24 h treatment periods for each. In addition, the percentage of abnormal cells and the CA/cell increased dose-dependently for 12 h treatment period; in contrast, mitotic index was decreased when compared with negative control and solvent controls for 12-h treatment period. However, mitotic index increased depending on tetra antibiotic dose for 24-h treatment period.