Abstract: A method of decreasing elevated levels of IL-6 and .gamma.-IFN that are secreted in response to traumatic injury in a warm-blooded animal is disclosed, comprising the step of administering an effective amount of melatonin to the animal. The melatonin should be administered so as to provide a peak amount of circulating melatonin at a time in the animal's daily cycle when endogenous melatonin reaches peak amounts. Elevated levels of IL-6 and .gamma.-IFN are also decreased by exposing the animal to certain daily regimens of light and dark, such as 24 hours of dark. Cell-mediated immune response to a contact antigen is also enhanced by administration of exogenous melatonin.