Parasites indirectly affect life-history evolution of most species. Combating parasites requires costly immune defenses that are assumed to trade off with other life-history traits. In vertebrate males, immune defense is thought to trade off with reproductive success, as androgens enhancing sexual signaling can suppress immunity. The phenotypic relationship between male androgen levels and immune function has been addressed in many experimental studies. However, these do not provide information on either intra- or intersex genetic correlations, necessary for understanding sexual and sexually antagonistic selection theories. We measured male and female humoral antibody responses to a novel antigen (bovine gamma globulin), total immunoglobulin G, and the male testosterone level of a laboratory population of the bank vole (Myodes glareolus). Although we studied five traits, factor-analytic modeling of the additive genetic (co)variance matrix within a restricted maximum likelihood-animal model supported genetic variation in three dimensions. Sixty-five percent of the genetic variation contrasted testosterone with both immune measures in both sexes; consequently, selection for the male trait (testosterone) will have correlated effects on the immune system not only in males but also in females. Thus, our study revealed an intra- and intersexual genetic trade-off between immunocompetence and male reproductive effort, of which only indirect evidence has existed so far.