Apoptosis (Ao) is a pathological process by which cells undergo a form of inducible nonnecrotic cellular suicide. In vitro studies suggest that changes in the rate of macrophage (Mø) Ao may be associated with elevated proinflammatory cytokine secretory capacity, such as interleukin-1 beta (IL-1β) (via IL-1 converting enzyme activation). Furthermore, it has been reported that Mø are activated during early (0-4 hours) experimental septic insult to act as sources of proinflammatory cytokines, such as IL-1. However, with the progression of sepsis, these same cells become refractory to further stimulation (appearing dysfunctional). Nonetheless, it remains unknown if this acquired immunosuppression (dysfunction) is associated with an acceleration in macrophage Ao. To determine this, male C3H/ HeN mice were subjected to sepsis (cecal ligation and puncture, CLP) or sham-CLP and 4 or 24 hours thereafter Mø were isolated from the peritoneum (PMø) and liver (KMø). Macrophage monolayers were lysed either after stimulation with lipopolysaccharide (LPS) (10 μg/mL, 24 hours) in vitro or immediately (ex vivo) before LPS stimulation and the cytoplasmic cell fraction was retained. The extent of Ao was determined using a cell-death enzyme-linked immunosorbent assay, which detects the presence of cytoplasmic oligonucleosomes and changes in the propidium iodide staining intensity. The results indicate that, early after CLP (4 hours) only PMø stimulated with LPS in vitro showed evidence of increasing Ao. At 24 hours (late) after the onset of sepsis, the ex vivo extent of Ao in PMø was increased but it was decreased in KMø. However, the addition of LPS in vitro results in a marked increase in both septic PMø and KMø Ao. This latter result suggests that the inability of Mø to release cytokines in response to stimulants, such as LPS during late sepsis (24 hours), may be because of induction of accelerated Ao in these Mø populations.