Background: Modulation of the immune response is an important strategy by which establish­ment and growth of hydatid cyst in the internal organs of human is warranted. Induction of apop­tosis in the lymphocytes might be a considerable component. This study was designed to evaluate apoptotic impact of hydatid fluid (HF) on human lymphocytes.Methods: Human lymphocytes were treated with hydatid fluid. After 6 hours of exposure, cas­pase-3 activity, the central enzyme of apoptosis cascade, was measured by fluorometric assay in the HF-treated lymphocytes and control cells. In addition, the expression of Bax (a pro-apoptotic protein) and Bcl-2 (an anti-apoptotic protein) mRNA was assessed by RT-PCR after 12 hours of exposure.Results: Both the ratio of Bax/Bcl-2 mRNA expression and Caspase-3 activity were higher in the HF-treated lymphocytes relative to the control group.Conclusion: Apoptosis could be as a possible mechanism by which Echinococcus granulosus overwhelms host defenses.