Contamination of water with arsenic is a problem affecting several regions of the world. Peripheral blood mononuclear cells (PBMC) from chronically exposed individuals show a lower replicating activity than non-exposed individuals when stimulated with phytohemagglutinin (PHA). We have previously reported that PBMC from healthy donors treated in vitro with 1 μM sodium arsenite (NaAsO 2) and stimulated with PHA showed a reduction in proliferation by a delay in cell cycle entry and a decrease in the rounds of cell division. In this paper we tested the effect of 1–5 μM NaAsO 2 on the proliferation, viability, blast transformation, expression of the CD4 and CD8 molecules, and during the activation and proliferation of both CD4 + and CD8 + T lymphocytes. We found a reduction in cell proliferation and an increase in non-dividing cells with higher concentrations of NaAsO 2 (2–5 μM) when proliferation was studied by 5,6-carboxyfluorescein diacetate succinimidyl ester (CFSE) dilution. The use of 7-aminoactinomycin D (7-AAD) in CFSE-labeled cells allowed us to detect an increase in percentage of non-dividing cells, and an increase in apoptotic/dead cells mainly in non-proliferating cells. Analysis of the expression of CD4 and CD8 molecules on these cells showed that concentrations ≥ 2 μM NaAsO 2 reduced the expression of the CD8 molecule and induced apoptosis/death in CD4 + cells. Analysis of blast transformation by flow cytometry showed an accumulation of CD8 + resting cells in the presence of NaAsO 2. Analysis of CD25 and CD69 expression in kinetics experiments in both subtypes showed a delay in the expression of CD25 and a delay in the downregulation of the CD69 molecule, in both CD4 + and CD8 + cells. However, in the case of CD8 + cells, we detected an accumulation of a CD25 −CD69 − population in the presence of increasing concentrations of NaAsO 2. Altogether, our results show that NaAsO 2 alters the expression kinetics of the early activation molecules CD25 and CD69 similarly in both subtypes. In addition, activated and non-activated CD4 + cells die by apoptotic mechanisms and although a percentage of CD8 + cells also die by apoptosis, a subpopulation of these cells is unable to activate and thus accumulates as resting cells.