Although interleukin-6 (IL-6) plays an important role in the pathophysiology of trauma-hemorrhage and resuscitation, the cellular origin of this inflammatory cytokine remains unknown. This study was undertaken to determine whether Kupffer cells (KC) are a major source of IL-6 release following trauma-hemorrhage and resuscitation. KC numbers were significantly (p < .05) reduced in vivo with gadolinium chloride (GdCl3; 10 mg/kg IV). KC-reduced (KC(-)) and KC-normal (saline-treated; KC(+)) rats underwent laparotomy (i.e., trauma-induced), followed by either sham operation or hemorrhage. Hemorrhaged rats were bled to and maintained at a mean arterial pressure of 40 mmHg until 40% of the shed blood volume was returned as Ringer's lactate, and then resuscitated with Ringer's lactate (four times shed blood volume over 1 h). Results indicate that KC reduction per se had no effect on any measured parameter at any time. At 0.5 and 2.0 h postresuscitation, mean arterial pressure, heart rate, cardiac output, stroke volume, and hematocrit were reduced to a similar extent in both the KC(+) and KC(-) hemorrhage groups. KC reduction did, however, significantly reduce plasma IL-6 concentration (means +/- S.E.; U/ml) at both 0.5 h (KC(+) = 709 +/- 391 vs. KC(-) = 159 +/- 5) and at 2.0 h (KC(+) = 527 +/- 394 vs. KC(-) = 83 +/- 20) postresuscitation. In conclusion, this study demonstrates that KC are a major source of in vivo IL-6 release following trauma-hemorrhage and resuscitation.