The indiscriminate use of anabolic androgenic steroids (AAS) has motivated researchers to investigate the mutagenic action of these substances. The present study, using the mouse bone marrow micronucleus test, evaluates the genotoxic potential of testosterone cypionate (deposteron). Male Swiss mice received intramuscular injections of deposteron at three doses. The animals were sacrificed 24, 48, or 72h after treatment and bone marrow was removed immediately, followed by scoring to count the micronuclei in 2000 polychromatic erythrocytes (PCE). Two hundred erythrocytes/animal were analyzed to determine the PCE–NCE (normochromatic erythrocyte) relationship and to determine the cytotoxic effects. The animals treated with deposteron at the highest dose presented greater numbers of micronuclei. The highest dose caused a decrease in the PCE/NCE relationship, indicating a cytotoxic effect. We conclude that deposteron is genotoxic and cytotoxic in mice.