In order to assess the role of osteopontin (OPN) in leukocyte accumulation in inflammatory conditions, native OPN and its thrombin cleaved form (OPN+Thr) were studied in vivo using a rodent subcutaneous air pouch model (AP). Both forms of OPN-induced macrophage infiltration into the AP in wild-type mice. In animals lacking CD44, macrophage numbers were significantly reduced within the cavity, but cells still accumulated along the subcutaneous lining. In animals lacking endogenous OPN, no differences were found in exogenous OPN-induced macrophage accumulation, although macrophage exhibited increased alpha4 integrin expression. These studies reveal that both OPN and OPN+Thr attract macrophages in vivo through CD44.