Examination of the response of injured patients' lymphocytes to the mitogen phytohemagglutin in a defined medium provides a mechanism to define the relationship between alteration in immune function and septic complications. Lymphocytes from 30 victims of gunshot wounds to the abdomen were examined. Response to mitogen was measured by incorporation of [3H]-thymidine as a function of lymphocyte concentration, with a constant amount of mitogen phytohemagglutinin and a standard incubation period. A saturation curve was obtained, and lymphocyte response was expressed as the concentration necessary for half-maximal incorporation of radioactive label, L1/2. Lymphocyte transformation was compared with that found in a group of 50 healthy volunteers. On arrival in the emergency center, the in vitro lymphocyte response of patients was markedly diminished. There were seven patients for whom a lymphocyte curve could not be generated, i.e., L1/2 greater than 1 X 10(6). For the other 23 patients, L1/2 = 4.75 X 10(5) (SEM - 7.5 X 10(4)) compared with L1/2 = 1.5 X 10(5) (SEM - 5 X 10(4)) for normal volunteers (p less than 0.01). Measurement of skin test response, white blood cell count, anthropometric measurements, and albumin level were not predictive of patient course. In contrast the in vitro lymphocyte viability corresponded to the degree of injury, and recovery of lymphocyte function was associated with improvement in the patient's clinical course.