To investigate alterations in host defense produced by trauma, skin testing with five standard recall antigens was done on admission and weekly on 53 patients with blunt trauma and seven with penetrating missile injuries, who then were classified as normal (N), 2 or more positive responses; relatively anergic (RA), one positive response; or anergic (A), no response. Neutrophil chemotaxis was tested 145 times in 32 patients. Degree of injury was assessed by assigning one point to pelvic fracture, long-bone fracture, head, chest, or abdominal injury, to a maximum of five. The A and RA patients had greater trauma, 3 vs. 1.6 for N, and a significantly increased rate of sepsis (p less than 0.005) and mortality (p less than 0.05). Incidence of anergy depended upon age and extent of trauma. Neutrophil chemotaxis in A and RA patients was significantly (p less than 0.001) worse at 96.7 +/- 2.4 mu and 99.8 +/- 1.7 mu compared to N, 113.2 +/- 1.7 mu, and controls 121 +/- 4 mu. With recovery, chemotaxis returned to normal. It is concluded that failure of delayed hypersensitivity responses follows trauma, is related to the severity of injury and age of patient, and is associated with an abnormality of neutrophil chemotaxis and increased rate of sepsis.