The antigenotoxic action of three doses of trans-dehydrocrotonin (t-DCTN), the active ingredient obtained from the bark extract of Croton cajucara, a plant native to the Amazon, was determined in Swiss mice in vivo. Mice were submitted to acute intraperitoneal and gavage treatments, then their bone marrow cells were subsequently analyzed by micronucleus (MN) and chromosome aberration (CA) assays. Comparisons were performed between the three doses of t-DCTN and the negative-control group. Statistical analysis indicated that doses of 50 and 75 % of the LD(50), via intraperitoneal treatment or gavage injection, were antimutagenic with regard to cyclophosphamide. However, the dose of 25 % of the LD(50) was only antimutagenic when administered by gavage. Based on these observations, it can be suggested that gavage is the most effective method of administering t-DCTN. In addition, t-DCTN showed no cytotoxic effects in the bone marrow cells regardless of the route of exposure.