Abstract The cytotoxicity of an alcoholic extract of Dendrostellera lessertii (Thymelaeaceae) and one of its purified components was evaluated using four different human cancer cell lines. The IC 50 of the crude extract was found to be 28, 33, 31 and 30 μl for K562, CCRF-CEM, HL-60 and LNCaP-FGC-10 cells, respectively. Each microliter of the crude extract corresponds to 0.02 mg of the plant powder material. Similarly the IC 50 of the purified component was 5×10 −9, 20×10 −9, 8×10 −9 and 8×10 −9 M for K562, CCRF-CEM, HL-60 and LNCaP-FGC-10 cells, respectively. In addition, besides significant reduction in cell proliferation rates, the crude extract, at a dose of 18 μl/ml and the purified component at a dose of 3.5×10 −9 M, were capable of enhancing the intracellular protein content of each of the cell lines by almost 160% while the intracellular alkaline phosphatase activity was increased up to 450%.