Background. The purpose of this study was to determine whether a decrease in macrophage energetics contributes to the profound immune dysfunction that occurs after hemorrhage and, if so, whether adenosine triphosphate (ATP)-MgCl2 treatment has any beneficial effects on the above parameters. Methods. C3H/HeN mice were bled to a mean blood pressure of 35 mm Hg, maintained at that pressure for 60 minutes, resuscitated with their own shed blood and Ringer's lactate, and treated with ATP-MgCl2 (80 μmol/kg body weight) or saline solution (vehicle). Peritoneal macrophages were harvested 1 hour after resuscitation and ATP levels were determined by 31P nuclear magnetic resonance spectrometry. In addition, macrophage functions were determined by measuring antigen presentation capacity (AP), as well as interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF) synthesis. Results. Hemorrhage caused a significant decrease in peritoneal macrophage AP function, as well as IL-1, IL-6, and TNF synthesis, by 52% ± 14%, 91% ± 12%, 78% ± 8%, and 89% ± 8%, respectively, which was correlated with a 78% ± 6% decrease in macrophage ATP levels (p < 0.05). Treatment with ATP-MgCl2 after hemorrhage restored macrophage ATP levels (p < 0.05) and significantly increased (p < 0.05) macrophage AP, IL-1, IL-6, and TNF release by 110% ± 21%, 130% ± 38%, 124% ± 17%, and 66% ± 24%, respectively. Conclusions. The decreased macrophage ATP levels may be the cause of defective macrophage AP and cytokine release after hemorrhage, and both macrophage ATP levels and macrophage immune functions can be restored with adjuvant ATP-MgCl2 treatment after hemorrhage. © 1992.