Despite the relevance of immune responses in cancer development, little is known about arsenite-induced apoptosis on different immune effectors cells. In this work, we determined the effect of in vitro exposure to sodium arsenite on apoptosis rates of blood lymphocytes, monocytes and NK cells. Blood was obtained from six healthy non-exposed donors, and also from a woman chronically exposed to arsenic. The results indicated that in vitro exposure of mononuclear cells (MNC) from non-exposed donors to sodium arsenite showed no increase in apoptosis as compared to non-treated cells. In contrast, cells obtained from the exposed-donor showed a significant increase in apoptosis after the treatment with sodium arsenite as compared to non-treated cells. This effect was observed in CD3+ and CD56+ but not in CD14+ cells. In addition, we found a preexisting high basal level of apoptosis in MNC from the exposed-donor. These results indicate that chronic exposure to arsenic increases the sensitivity of immune cells to in vitro sodium arsenite-mediated apoptosis.