Two myricetin derivatives, the 3,7, 4('), 5(')- tetramethyl ether of myricetin (1), isolated from the hexane extract of Cistus monspeliensis, and its 3('),5-diacetyl derivative (2) which was synthesized, and the parent compound, myricetin (3), were examined for their in vitro cytotoxic activity against nine human leukemic cell lines, two of which were mdr cell lines. Compound 2 exhibited higher cytostatic and cytotoxic activities in comparison to compound 1, while compound 3 was inactive against all tested cell lines. Vinblastine was used as a control.