The studies described here demonstrate that the activation of the physiologic stress response systems of the body can enhance immune function in vivo. This enhancement is observed as a large and long lasting increase in allergic contact sensitivity or delayed-type hypersensitivity, an immune reaction which involves an Ag-specific, cell-mediated immune response. In contrast, acute stress has no effect on the course of irritant contact sensitivity, an immune reaction that does not involve an Ag-specific memory response. A comparison of infiltrating leukocyte numbers in sections of inflamed skin from unstressed and stressed animals shows that stress induces a significant and persistent increase in numbers of leukocytes at the site of the delayed-type hypersensitivity reaction. These results demonstrate that a relatively mild behavioral manipulation can enhance an important class of immune responses that mediate harmful (allergic dermatitis) as well as beneficial (resistance to certain viruses, bacteria, and tumors) aspects of immune function. The implications that these studies have for clinical, diagnostic, and experimental manipulations involving cell-mediated immune function are discussed.