In Brazil Croton cajucara Benth - (Euphorbiaceae), popularly known as “Sacaca” is used in popular medicine as a cytoprotective agent for gastric ulcers and to treat several illnesses such as diarrhea, diabetes and liver inflammation. Despite the good pharmacologic activity of this plant the use of this bark extract can induce liver damage based on in vivo evaluation. Experimental studies have consistently incriminated this medicinal plant in epidemic and sporadic cases of liver diseases. Based on these evidences, the aim of this study was to evaluate the toxic effects of Sacaca extract on Hamster Chinese fibroblast (V79) cells and in primary hepatocyte culture. Different biomarkers were used for relative viability assessment: MTT dye reduction and neutral red uptake (NRU). The lyophilised extract was obtained from Sacaca barks infusion (1.2 kg of powdered barks in one water liter). The viability assays used included lysosomal (NRU) and mitochondrial membrane integrity (MTT), which provides information about the susceptibility of cellular organelles and compartments. The IC50 values determined on V79 cells and hepatocytes were around 1 mg/mL evaluated by MTT reduction assay. It was observed an increasing in the neutral red uptake in hepatocytes treated with the extract (until 12 mg/mL). Inhibition of cytochrome P450IIIA in cultured hepatocytes by incubation with 1 mM cimetidine altered the toxicity of the extract in these cells. In conclusion, these observations suggest that P450-mediated biotransformation of this extract may lead to the formation of more toxic metabolites corroborating with liver injury verified in vivo.  Financial Support: FAPESP, Prometro.