ABSTRACT The essential oil from Croton polyandrus Spreng., Euphorbiaceae, leaves was tested for the toxicity and antitumor activity. The concentration producing 50% hemolysis was 141 µg/ml on mice erythrocytes. In the acute toxicological study, the estimated LD50 was 447.18 mg/kg. The essential oil did not induce increase in number of micronucleated erythrocytes, suggesting low genotoxicity. Essential oil (100 or 150 mg/kg) showed significant antitumor activity in Ehrlich ascitic carcinoma model. We observed that essential oil induces cell-cycle arrest at the G0/G1 phase, and increases the sub-G1 peak, which represents a marker of cell death by apoptosis. Survival also increased for the treated animals. The toxicological analyses revealed reduction in body weight, increased aspartate aminotransferase and alanine aminotransferase activity, hematological changes, and a thymus index reduction. These data suggest gastrointestinal and liver toxicity, anemia, leukopenia/lymphocytopenia, and immunosuppressive effects. Histopathological analysis revealed the weak hepatotoxicity of essential oil. In summary, essential oil of C. polyandrus displays in vivo antitumor activity and moderate toxicity.