Pneumonia in the critically ill surgical patient often results from the bombardment of a previously normal pulmonary system with therapeutic foreign bodies, hospital pathogens, and impairment of the host defenses. Despite its long history as a significant clinical problem, a woefully inadequate amount of study has been directed toward therapy. We created an experimental model of a differential pulmonary infection using a strain of Klebsiella pneumoniae. We then compared the progressively affected pneumonic process versus the normal parenchyma. We measured neutrophil and monocyte complement antibody receptor expression and monocyte and macrophage class II major histocompatibility antigens (HLA-DR) via percent of cells and mean fluorescent intensity outcomes from flow cytometry. The main difference between infected versus noninfected tissues was monocyte DR expression, which was consistently depressed in cells from infected parenchyma. What follows is a discussion of the implications of this work as well as other work in the immunology of pneumonia and cytokine expression. Possible therapeutic modalities are included.