Ionized calcium-binding adapter molecule 1 (Iba1) is a 147-amino-acid calcium-binding protein widely in use as a marker for microglia. It has actin-crosslinking activity and is involved in aspects of motility-associated rearrangement of the actin cytoskeleton. The Iba1 gene and protein are identical to allograft inflammatory factor-1 (AIF-1), a protein involved in various aspects of inflammation, which was investigated independently from Iba1. Although regarded to be monocyte/macrophage-specific, expression by germ cells in testis showed that AIF-1/Iba1 is not exclusively expressed by cells of the monocyte/macrophage lineage. Furthermore, AIF-1 was found in cells not belonging to the monocyte/macrophage lineage under pathological conditions. Here, the distribution of AIF-1/Iba1 in the normal mouse has been examined, by immunohistochemistry, to determine whether AIF-1/Iba1 expression is confined to macrophages and spermatids. Spermatids are the only cells not belonging to the monocyte/macrophage lineage found to express AIF-1/Iba1 in the normal mouse, by this method. This study has not demonstrated AIF-1/Iba1 expression in dendritic cells, although this protein might be expressed by subsets of dendritic cells. AIF-1/Iba1 can be regarded a "pan-macrophage marker" because, except for alveolar macrophages, all subpopulations of macrophages examined express AIF-1/Iba1.