Intact peritoneal macrophage (Mø) function is critical to successful localization of intra-abdominal infection. Peritoneal macrophage antigen presentation capacity (APC), interleukin-1 (IL-1) expression, and immune-associated (la) antigen expression and abscess formation were determined following cecal ligation and puncture. APC and IL-1 expression were measured by coculture with a T-helper cell clone and by measuring subsequent proliferation, la expression was determined in blood, peritoneal Mø, and splenocytes using anti-la monoclonal antibody stain and flow cytometric analysis. Significant reductions in both la expression and APC were found 1 and 4 days after CLP with no change in IL-1 expression. Muramyl dipeptide, which enhances Mø phagocytosis, partially abrogated the depression in antigen presentation but did not affect la expression. Peritoneal Mø la expression and APC, but not IL-1 expression, were depressed after experimental peritonitis. The recovery of Mø function by day 14 coincides with clinical recovery and abscess formation, and restoration of early Mø depression may improve outcome.