M any inflammatory processes are characterized by an early phase of neutrophil migration and a later phase of monocyte migration into the inflammatory site. Mechanisms that govern the transition between phases are the subject of these investigations. Acute lung inflammation induced by C5 fragments in the rabbit leads to an initial neutrophil influx and plasma leakage into the alveolar space, followed by monocyte influx that we have previously shown to be dependent on prior emigration of neutrophils. Neutrophil enzymes are known to cleave intact fibronectin into fragments that are monocyte chemotaxins in vitro. Accordingly, generation of appropriate fibronectin fragments in situ by proteolytic enzymes from infiltrating neutrophils might represent a potential mechanism for attraction of monocytes into the lung. The studies reported herein demonstrate that a 120-kD fragment of fibronectin containing the RGDS fibroblast cell-binding domain induced monocyte migration into the rabbit lung in vivo. Intact fibronectin was inactive. A significant proportion of the monocyte migration was neutrophil independent. Intact fibronectin was present in bronchoalveolar lavage fluid from C5 fragment-treated animals rendered neutropenic, but absent in lavage from normal C5 fragment-treated animals. Fibronectin fragments were present in bronchoalveolar lavage fluid from both C5 fragment-treated and control rabbits. In addition, the amount of fibronectin was significantly increased in lavage of C5 fragment-treated normal but not neutropenic animals. Monoclonal antibodies directed against an epitope of fibronectin containing the RGDS cell-binding domain significantly inhibited the C5 fragment-induced monocyte migration, but not neutrophil migration. These studies suggest that chemotactic fibronectin fragments may in part be responsible for the recruitment of monocytes into areas of acute lung inflammation.