Microglial cells are among the first and dominant cell types to respond to CNS injury. Following calcium influx, microglial activation leads to a variety of cellular responses, such as proliferation and release of cytotoxic and neurotrophic mediators. Allograft inflammatory factor-1, AIF-1 is a highly conserved EF-handed, putative calcium binding peptide, associated with microglia activation in the brain. Here, we have analyzed the expression of AIF-1 following spinal cord injury at the lesion site and at remote brain regions. Following spinal cord injury, AIF-1+ cells accumulated in parenchymal pan-necrotic areas and perivascular Virchow-Robin spaces. Subsequent to culmination at day 3--a situation characterized by infiltrating blood borne macrophages and microglia activation--AIF-1+ cell numbers decreased until day 7. In remote areas of Wallerian degeneration and delayed neuronal death, a more discrete and delayed activation pattern of AIF-1+ microglia/macrophages reaching maximum levels at day 14 was observed. There was a considerable match between AIF-1+ cells and PCNA (proliferating cell nuclear antigen) or Ki-67+ labeled cells. AIF-1 expression preceded the expression of ED1, thus indicating a pre-phagocytic role. It appears that AIF-1+ microglia/macrophages are among the earliest cells to respond to spinal cord injury. Our results suggest a role of AIF-1 in the initiation of the early microglial response leading to activation and proliferation essential for the acute response to CNS injury. AIF-1 might modulate microgliosis influencing the efficacy of tissue debris removal, myelin degradation, recruitment of oligodendrocytes and re-organisation of the CNS architecture.