Neutrophils (PMN) accumulate and are associated with cerebrovascular disturbances after experimental traumatic or ischemic brain injury, and meningitis. We hypothesized that posttraumatic PMN accumulation in brain is mediated by the PMN adhesion receptor Mac-1 (CD11b/CD18). Anesthetized rats were randomized to receive 2 mg/kg intravenously of murine monoclonal antibody to rat Mac-1 (1-B6) or anti-Mac-1 F(ab)2' [1-B6F(ab)2'] fragment (Repligen Corp., Cambridge, MA). Control rats were treated with isotype matched control antibody. Rats were subjected to percussive trauma to the right parietal cortex 30 min after treatment. Rats were killed 24 h posttrauma, and PMN accumulation was assessed by myeloperoxidase (MPO) activity. The presence of 1-B6F(ab)2' bound to PMN in brain after trauma was assessed by immunohistochemistry. Complete blood cell counts were obtained before treatment and 24 h after trauma. Brain MPO activity was reduced by 43% in the 1-B6-treated rats vs. controls (0.31 +/- 0.09 vs 0.55 +/- 0.10 U/g, n = 6/group, p = 0.013) and by 34% in the 1-B6F(ab)2'-treated rats vs. controls (0.43 +/- 0.10 vs. 0.65 +/- 0.09 U/g, n = 6/group, p = 0.006). Systemic neutropenia developed in the 1-B6-treated rats (absolute PMN count decreased by 73% vs. baseline) but not in rats treated with 1-B6F(ab)2' (absolute PMN count increased by 26 and 25% vs. baseline in treated and controls, respectively). Immunohistochemical staining showed 1-B6F(ab)2' on the surface of infiltrated PMN 24 h after trauma. Mac-1 mediates posttraumatic PMN accumulation in brain. This accumulation can be attenuated by 34%, without reducing circulating PMN, using an anti-Mac-1 F(ab)2' fragment; however, some PMN coated with 1-B6F(ab)2' still infiltrate into traumatized tissue. These results are similar to those reported in models of cerebral ischemia, and suggest the participation of multiple PMN adhesion pathways after ischemic and traumatic brain injury.