Cuphea aequipetala (Lytraceae) is a perennial plant that has been used in Mexican traditional medicine to treat different types of tumors since prehispanic times. In the present work the cytotoxic potential of different fractions from acetone-water extract from the whole plant was investigated using a sulforhodamine B assay. Fractions were subjected to a bioscreening assay using several cell lines: HEp-2 (human larynx carcinoma), HCT-15 (human colon cancer) and DU-145 (human prostate carcinoma). Colchicine was used as positive control. Data are presented as the dose that inhibited 50.0% control growth (ED50). The cytotoxic activity is selective since the ED50 is different for the three cell lines employed. The highest activity was seen against the DU-145 cell line. "E" and PB1 fractions had the highest cytotoxic activities with ED50 values of 0.418 and 2.40 microg/ml respectively, on the DU-145 cell line. The "E" fraction was a yellow powder; it was methanol soluble and contained at least four separate components when separated by thin-layer chromatography. PB1 was a solid with metallic appearance; it was water soluble and its two dimensional chromatography showed 9 spots. These fractions have cytotoxic actives because their ED50 is less than 20 microg/ml and they will be further characterized.