Although it is known that decreased high-energy phosphates contribute to organ dysfunction following shock, it remains unknown whether changes in lymphocyte energetics contribute to the profound immune dysfunction that occurs in late septic shock. Moreover, while studies have shown that ATP- MgCl2 treatment after hemorrhagic shock improves tissue ATP levels and organ function, it remains unknown whether lymphocyte high-energy phosphates and immune functions are similarly affected by this agent after sepsis. To study this, sepsis was induced in C3H/HeN (endotoxin sensitive) mice by cecal ligation and puncture (CLP) and they were then treated intraperitoneally with ATP-MgCl2 or saline vehicle. Sham animals received laparotomy, but not CLP. Splenic lymphocytes were harvested 24 hr after treatment and ATP levels were determined by ultraresolution 31P NMR. Lymphocyte proliferative capacity was determined by [3H]thymidine incorporation following mitogenic stimulation. Host survival was assessed following CLP with and without ATP- MgCl2 treatment. Prolonged sepsis caused a significant decrease (↓ 67 ± 12% vs Sham) in lymphocyte ATP levels which were correlated with decreased lymphocyte proliferative capacity in response to mitogenic stimulation (64 ± 17 x 103 vs. 232 ± 43 × 103 counts per minute (cpm) in Sham; P < 0.05). Treatment with ATP-MgCl2 at the onset of sepsis significantly increased lymphocyte ATP levels (↑ 32 ± 15% vs CLP) and proliferative response to mitogenic stimuli (218 ± 37 × 103 cpm, CLP/ATP-MgCl2; P < 0.05). Improved lymphocyte function in this group correlated with a significant increase in overall survival (20% CLP vs 70% CLP/ATP-MgCl2; P < 0.05). We conclude that: (1) decreased lymphocyte ATP levels may be the cause of defective lymphocyte proliferative capacity in late sepsis, (2) adjuvant treatment with ATP- MgCl2 improves both lymphocyte ATP levels and lymphocyte proliferative capacity, and (3) intraperitoneal ATP-MgCl2 decreases lethality from sepsis. © 1994 Academic Press, Inc.