Leukemias are malignant neoplasms that affect blood cells and originate in bone marrow. These are cancers of high incidence and are difficult to treat and there is a constant search for more efficient therapies. The present work investigated the cytotoxic action of the hydroalcoholic extract of Croton urucurana in human leukemia cells U937 and THP-1. The U937 and THP-1 cell lines were plated in a 100 μL/well volume (1x106 cells/mL) in 96-well plates, treated with extracts at final

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... of 50 μg/mL, 100 μg/mL, 200 μg/mL and 400 μg/mL and 800 μg/mL for cell viability tests with MTT(-3- (4.5-dimethyl-2-thiazole) 2.5-diphenyl-2H-tetrazolium bromide). After 48h, viability was assessed by MTT and the supernatant was used for determination of the Lactate Dehydrogenase (LD) enzyme. Fluorescence microscopy was used to evaluate apoptosis. The extract was able to reduce cell viability by the MTT assay, increase LD release and induce apoptosis in both cell lines tested at concentrations of 400 and 800 μg/ mL (p≤0.05). The C. urucurana extract presented cytotoxic activity against human leukemic cell lines U937 and THP-1, presenting itself as promising in the search for new antineoplastic agents.