Immunoincompetency is often seen in patients after various types of trauma and is associated with increased morbidity and mortality from infectious complications. To understand better the immunologic impairment associated with trauma, we have studied this phenomenon in an animal model. Splenocytes from mice traumatized by amputation of their right hind limbs were consistently shown to have a diminished capacity to proliferate in response to alloantigens and to form alloreactive cytolytic cells in mixed lymphocyte cultures. Anesthesia itself had no effect in this system. The immunoincompetency was detected from 2 h to 6 d after surgical trauma and was completely reversed by removing adherent and phagocytic cells from the splenocytes. Furthermore, addition of splenocytes from traumatized mice to mixed lymphocyte cultures from normal mice prevented normal lymphocytes from responding to alloantigens, suggesting the existence of suppressor cells. The suppressor cells were found to adhere to glass and to nylon wool columns, and were contained within an esterase-positive cell population. They were insensitive to treatment with anti-Thy 1.2 and anti-Ig sera in the presence of complement. Therefore, the present results suggest that a Thy 1.2-negative, Ig-negative, esterase-positive cell population capable of adhering to glass and nylon wool, presumably macrophages, was responsible for the inhibition of the responsiveness of lymphocytes to alloantigens in traumatized animals.