Sepsis is responsible for 75% of late deaths after major thermal or traumatic injury. In the clinical setting, efforts to prevent or control sepsis, or both, should include an understanding of normal host resistance, proper resuscitation techniques, and nutritional support. Previous studies have identified T suppressor cell abnormalities following thermal injury and have suggested macrophage defects after traumatic injury. Although major thermal injury is easier to quantify than mechanical trauma, both insults can stress patients' host resistance to a maximal degree, leading to profound and often fatal immunosuppression. Recent studies summarized in this paper have suggested that the macrophage and the interleukin system may play a major role in initiating some of these immune abnormalities following injury.