The ability of the rodent testis to tolerate graft alloantigens and spermatogenic cell autoantigens is well known. The mechanisms underlying this immune privilege are poorly understood, but the numerous resident TMs have been implicated. Although it has been assumed that TMs display a phenotype consistent with immune privilege, this has not been formally established. Consequently, TMs were isolated from adult rats and cultured under basal conditions and following stimulation with LPS and IFN-gamma (classical activation) or IL-4 (alternative activation). BMMs matured in vitro were used as control. Expression of the classical (proinflammatory) activation markers TNF-alpha, IL-1beta, iNOS, IL-6, RANTES, IL-12p40, and SOCS3 and alternative (immunoregulatory) activation markers IL-10, TGF-beta1, CXCL2, and SOCS1 was measured by QPCR or ELISA. In culture, TMs were characterized by poor expression of classical activation genes and TGF-beta1 but constitutively high IL-10 production and reduced costimulatory activity in a polyclonal T cell activation assay. This pattern of gene expression was associated with TMs expressing the scavenger receptor CD163, which is characteristic of tissue resident macrophages and alternative activation. By contrast, CD163-negative TMs displayed reduced inflammatory gene expression but did not constitutively produce IL-10. These data indicate that under the influence of the testicular environment, macrophages adopt an alternatively activated phenotype, involving reduced capacity for proinflammatory gene expression, constitutive IL-10 production, and impaired ability to support T cell activation, consistent with a role in maintaining testicular immune privilege.