Reperfusion of ischemic tissue is associated with tissue injury greater than that resulting from ischemia alone. C activation has been hypothesized to mediate the so-called ischemia/reperfusion injury through both membrane attack and C5a-dependent recruitment of neutrophils to sites of C3 fixation on the endothelium via C3 receptors. Adherence of neutrophils is preconditional to expression of their deleterious effects, which are central to the pathophysiology of ischemia/reperfusion injury. This study was designed to evaluate the effect of inhibition of C activation on ischemia/reperfusion injury using a soluble and truncated recombinant human CR1 (sCR1) molecule, a "tail-less" form of the membrane C3b/C4b receptor (CD35) that functions as a regulator of C activation. Capillary perfusion and leukocyte adherence to venular endothelium were measured after reperfusion in a mouse cremaster muscle model that allowed microscopic video observation of microcirculatory changes. Infusion i.v. with sCR1 before a 4-h period of ischemia and during a 3-h subsequent period of reperfusion prevented the increase in leukocyte adherence to venular endothelium seen in controls, and enhanced the number of reperfusing capillaries by 55%. Trypan blue staining showed an increase in muscle cell viability from 11 to 50% in mice receiving sCR1 as compared to controls. Tests of blood samples from mice infused with sCR1 demonstrated nearly complete inhibition of the mouse alternative pathway of C activation, but no detectable loss of the mouse classical pathway of C activation. It was concluded that C activation in this model of skeletal muscle injury is likely to be due to the alternative pathway, and that inhibition of C activation during reperfusion inhibits leukocyte adherence to blood vessel walls and protects the capillary microcirculation.