Although departures from the normal physiologic pH range, associated with states of shock, injury, and/or infection are thought to impair phagocytic function and chemotaxis, it remains unknown if the ability of the peritoneal macrophage (pMφ) to gate H+ is affected following hemorrhagic shock. To study this, male C3H/HeN mice were bled to a BP of 40 mm Hg and maintained at that level for 1 hr. Following resuscitation with the shed blood plus 2x that volume as Ringer’s lactate, pMφ was harvested via lavage in bicarbonate- free RPMI 1640 and enumerated to 2 million cells/ml. PMφ was then loaded with a pH-sensitive dye, BCECF (2 μg/ml), for 20 min and washed with a buffered Na+ solution followed by phorbol myristate acetate stimulation (0.5 μg/ml). The capacity of the pMφ to gate H+ at an external pH of 7.35 or 6.7 was assessed via fluorescence spectrophotometry. PMφ from hemorrhaged animals had a decreased ability to alkalinize intracellular pH compared to that of control (P < 0.05), indicating a decreased ability of these Mφ to maintain internal pR in an acidic environment following hemorrhage. In additional studies, oxidative burst capacity was determined by employing a commercially available fluorogenic substrate, (Fc Oxiburst), which fluoresces during intracellular oxidation. Fluorescence (U × 106) was assessed with the ACAS 570 laser cytometer, after 0 and 200 sec following ingestion of substrate. The results demonstrate that pMφ oxidative burst capacity was also decreased following hemorrhage and resuscitation. We propose that the underlying mechanism of decreased ability of pMφ to alkanize intracellular pH and oxidative burst capacity may be due to loss of intracellular ATP following hemorrhage. This cellular derangement may be an important factor contributing to the depressed cellular immune function under such conditions. © 1995 Academic Press, Inc.