BACKGROUND: Hydatid cyst is an infection with global distribution that is caused by the larval stage of the tapeworm of Echinococcus. The long-term survival of the hydatid in the host shows the parasite has advanced highly effective strategies for escaping the host defense. Deaths are caused by parasitic infections which are often due to tissue damages that result in host cell death, this is known as apoptosis. So it is important to know the process and the role of apoptosis that is created or controlled by a parasite. OBJECTIVES: Investigation of cytotoxicity effect, induction of apoptosis and mechanism of induction of apoptosis of cattle hydatid fluid on bovine lymphocyte cells as efficient cells of immunity were studied. METHODS: In this study, the cytotoxicity effect of bovine hydatid fluid (HF) on lymphocyte cells was investigated as effective immune cells against Echinococcus granulosus by MTS method. Then the mean of the expression of caspase-3, Bax and Bcl-2 genes in bovine lymphocytes treated/untreated was determined with fertile and infertile hydatid cyst fluid using Real Time PCR method. RESULTS: The viability mean of lymphocytes was significantly lower in the fertile HF treated lymphocytes compared to both infertile HF-treated lymphocytes and cell control. Bax gene expression was significantly (P=0.046) higher in the fertile HF-treated lymphocytes compared to both infertile HF-treated lymphocytes and cell control. Although Caspase 3 was higher in this group, the difference was not significant. Also, expression of Bcl-2 gene in fertile fluid treated lymphocytes was found to be lower than that of infertile and control. CONCLUSIONS: Present study indicates that hydatid cyst fluid molecules can probably induce apoptosis in immune cells in vitro and the parasite’s ability to stay alive for a long time in the host by controlling the host immune response from the apoptosis pathway.