For many years, in vitro and in vivo studies have reported that organosulfur compounds (OSCs), naturally found in Allium vegetables, are able to suppress the proliferation of various tumor cells. In spite of recent advances, the specific molecular mechanisms involved in OSC activity are still unclear. Considering the antiproliferative effects observed in cancer cells, we postulated that OSCs might target the cell division cycle (Cdc) 25 phosphatases which are crucial enzymes of the cell cycle. Our findings suggest phosphatases Cdc25 as possible targets of naturally occuring polysulfides contributing to their anticancer properties. We report on the inhibitory activity of tetrasulfides occurring naturally in garlic and onion towards the human Cdc25 phosphatases. Diallyl- and dipropyltetrasulfides have emerged as interesting irreversible inhibitors of the Cdc25 isoforms A and C in vitro. Furthermore, growth of both sensitive (MCF-7) and resistant (Vcr-R) human breast carcinoma cells was significantly decreased by these tetrasulfides. The observed antiproliferative effect appeared to be associated with a G2-M cell cycle arrest.