This study examined a group of surgical patients with respect to the ability of their peripheral blood mononuclear cells to respond to phytohemagglutinin (PHA). Depression of the PHA response of more than 30% below baseline five to seven days after injury was found in 11 of 19 patients, and eight of them developed infectious complications. The addition of indomethacin to in vitro cultures resulted in an average enhancement of the PHA response of 37% baseline. Improvement at five to seven days with in vitro indomethacin was from 34% to 74% in infected patients. These data suggest that major injury can lead to depression of the PHA response, which correlates with the subsequent development of infectious complications. Indomethacin in vitro seems to be able to reverse or decrease this immunologic defect and deserves further study. © 1986, American Medical Association. All rights reserved.