We tested the antiproliferative effects of Diheptyl Diselenide (DHDSe) on several different human cancer cell lines. Cells derived from human cancer (CG5), colon cancer (WIDR), laryngeal cancer (Hep-2), ovarian cancer (OV 166, OV 1225) and IM-9 lymphoblastoid cells were used. In all cell lines DHDSe inhibited cell growth in a dose dependent manner. At the highest concentration tested, an inhibition of cell proliferation ranging from 48% to 75% compared with control cells was observed. Our results show that DHDSe exerts a direct antiproliferative effect on human cancer cells in vitro and suggest that it may represent the parent compound of a new group of anticancer agents.