Macrophage (Mφ) prostaglandin (PG)E2 production has been implicated in immunosuppression and increased susceptibility to sepsis after thermal injury. Deficient interleukin (IL)-12 production has also been implicated in these postburn complications. The present study examined the relationship between Mφ cyclooxygenase (COX)-2 activity and IL-12 production after thermal injury. C57BL/6 female mice were subjected to a 25% total body surface area full-thickness burn. Mφ were isolated 7 days later, or the mice were subjected to sepsis by cecal ligation and puncture (CLP). IL-12 production by Mφ from injured mice was suppressed by >50%, whereas COX-2 expression and PGE2 production were increased twofold. The COX-2 inhibitor NS-398 suppressed PGE2 production and normalized IL-12 production in the injury group, whereas it had no effect on IL-10 production. Injured mice subjected to CLP had lower IL-12 plasma levels compared with sham-treated mice subjected to CLP. NS-398 treatment prevented the suppression in plasma IL-12 levels in the injury group. Thus elevated Mφ COX-2 activity, independent of IL-10, suppresses Mφ IL-12 production after thermal injury and may play an important role in the observed immunosuppression under such conditions.