The recruitment of leukocytes after administration of heme, saline, or SnMP. Both infiltrated granulocytes (A) and macrophages (B) are highly up-regulated after administration of heme (arrows). Inhibition of HO activity using SnMP exacerbated heme-induced leukocyte infiltration (original magnification, × 100). In some cases HO inhibition using SnMP alone already caused leukocyte infiltration (data not shown).