Recent studies indicate that depressed macrophage (Mφ) immune responses following hemorrhage can be restored by the administration of interferon (IFN)-γ as an adjuvant to resuscitation. However, the mechanism of these effects remains unknown. An important component of the process of Mφ activation in response to pathogens is the mobilization of intracellular calcium ([Ca2+](i)). Since hemorrhage alters the Mφ signal transduction system, the aim of this study, therefore, was to determine whether administration of IFN-γ after hemorrhage restores this mode of macrophage signal transduction. To assess this, C3H/HeN mice were bled to and maintained at a mean blood pressure of 35 mm Hg for 1 hr and then adequately resuscitated. Mice then received either 40,000 units IFN-γ/kg body wt or saline vehicle and were killed 2 hr posthemorrhage to obtain splenic Mφ. These Mφ were loaded with Fluo-3 AM (fluorescent [Ca+2](i) probe) and their capacity to mobilize [Ca2+](i) in response to stimulation with f- met-leu-phe (FMLP; bacterial peptide) was assessed on a laser cytometer. The results indicated that IFN-γ restored the capacity of splenic Mφ to mobilize [Ca2+](i) in response to FMLP. Moreover, the results demonstrated that hemorrhage produced a marked increase in resting cAMP levels in these cells that were lowered to sham levels by the administration of IFN-γ. Thus, it appears that IFN-γ acts to restore Mφ signal transductional capacity, thereby improving Mφ-mediated immune functions following hemorrhage and resuscitation. © 1993 Academic Press. All rights reserved.