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Heroin treatment or abusive drug addiction influences many physiological functions, including the reactions of the immune system. Although suppression of various manifestations of the immune system after heroin (or morphine) administration has been reported, we show here that production of proinflammatory cytokines and nitric oxide (NO) was enhanced and allotransplantation reactions were accelerated significantly in heroin-treated recipients. Mice were treated by a subcutaneous administration of heroin (diacetylmorphine) given in one or repeated daily doses. The ability of spleen cells from treated mice to respond in vitro to alloantigens and to produce IL-2, IL-4, IL-10 and IFN-γ, and the production of IL-1β, IL-12 and NO by peritoneal macrophages, were tested. Within 2 h after heroin administration, proliferative responses to alloantigens and the production of IL-1β, IFN-γ, IL-12 and NO were enhanced significantly. In contrast, the production of anti-inflammatory cytokines IL-4 and IL-10 was at the same time rather decreased. As a consequence, skin allografts in heroin-treated mice were rejected more promptly than in untreated or vehicle-treated recipients. Similarly, the growth of allogeneic tumours induced by high doses of tumour cells was suppressed significantly in heroin-treated mice. The enhancing effects of heroin on the production of proinflammatory cytokines were antagonized by naltrexone, a specific inhibitor of classic opioid receptors. These results show that heroin treatment augments production of proinflammatory cytokines and accelerates allotransplantation reactions. The observations thus illustrate the complexity of the effects of heroin on the immune system and should be taken into account during medical treatment of opiate addicts and in the use of morphine to decrease pain in various clinical situations.