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Background & Objective: Few studies concerning the effects of Opioid drugs on the function of immune system have been conducted and conflicting results have been reported. This study evaluates the in-vitro immune responses of drug abusers and investigates the pattern of production of IFN-? and IL-10, which represents the subsets of CD4+T-helper cells. Materials & Methods: Blood samples were taken from healthy drug addicted volunteers. Blood samples were also taken from healthy individuals with no history of drug abuse as control. Cell culture was performed in whole blood culture assay. Diluted blood samples were stimulated with phytohemagglutinin (PHA) and lipopolysaccharide (LPS) and the supernatants were collected to measure the Cytokine production. Results: The results demonstrated that a significant decrease in IFN-? production and increase in IL-10 production in Heroin addicts, whereas the production of these Cytokines in Opium addicts was not significant different from those in control group. Conclusion: The results indicated a significant decrease in mitogenic responsiveness of T-cells in Heroin addicts relative to control group, whereas mitogenic responsiveness of T-cells in Opium addicts was not significantly different from control group.