Th2 responses, especially IL-10 secretion by circulating mononuclear cells are associated with the progressive form of AE and Th1 responses with resistance. The HLA B8, DR3, DQ2 haplotype is associated with the severity of AE in humans through immune-mediated mechanisms including an elevated production of Interleukin-10 (IL-10). Granulomatous infiltration of mononuclear cells around the parasitic vesicles is a hallmark of this disease; however, cytokine production by granuloma cells has never been studied. Tissue samples were taken in the periparasitic area and in the central area of the periparasitic granulomatous lesions from a patient with a progressive AE at surgery. Six pieces for each zone were incubated in culture medium with antibiotics and IL-2, together with irradiated autologous peripheral blood mononuclear cells as feeder cells. After four days the dead feeder cells were removed by density gradient centrifugation. Lymphocytes were stimulated with Echinococcus multilocularis vesicular fluid antigen (Emf) or PHA to study IL-10, IFN-, and IL-4 production in the supernatant. Emf-stimulated mononuclear cells from the central part of the lesions secreted more IL-10 and less IFN-gamma than cells from the periphery of the granuloma. At the basal level, IL-10 secretion by the locally infiltrating cells was also high and this confirms at the local granuloma level our previous results obtained from cultures of circulating mononuclear cells. The present study confirms that IL-10 secretion is a key feature of the immune response against E. multilocularis in humans. The location of the cells which produce the highest amount of IL-10, those in contact with parasitic structures, suggests that the parasite itself is able to modulate the immune response of the host so that the infiltrating cells cannot participate in the effector phase of the cellular immune response. The nature of the parasitic structures involved and the mechanisms which lead to an imbalanced cytokine production remain to be elucidated.