Understanding the cost of immune function is essential for more accurate characterization of energy budgets of animals and better understanding of the role of immunity in the evolution of life-history strategies. We examined the energetic cost of maintaining a normally functioning immune system and mounting a mild immune response in wild male white-footed mice (Peromyscus leucopus). To evaluate the cost of maintaining immunocompetence, we compared resting and daily metabolic rates (RMR; DMR) and masses of body organs of mice whose immune systems were suppressed by cyclophosphamide with those of control mice. To evaluate the cost of mounting an immune response, we measured RMR, DMR, and organ masses in mice whose humoral and cell-mediated immune responses had been stimulated by injections of sheep red blood cells and phytohemagglutinin, respectively. Immunosuppression resulted in a significant reduction in circulating leukocytes, by 225%, but no significant effect on metabolic rates or organ masses. Immunochallenged animals showed no significant differences in metabolic rates compared with control animals but did exhibit significantly smaller dry masses of the small intestine and testes, by 74% and 22%, respectively. We concluded that the cost of maintaining the immune system was minimal. In contrast, there was a significant energetic cost of mounting an immune response that, depending on its magnitude, can be met through reductions in energy allocation to other physiological systems.