Using a well-characterized rat model of immune complex-mediated acute inflammatory lung injury, we determined that there is a time-dependent elaboration of monocyte chemotactic activity in bronchoalveolar lavage fluid. Monocyte chemotactic activity is also significantly enhanced in culture supernatants from pulmonary alveolar macrophages (PAMs) from injured rat lungs. Northern hybridization analysis revealed markedly increased expression of rat monocyte chemoattractant protein 1 (MCP-1) mRNA in PAMs obtained from rats with immune complex-induced lung injury. The increased expression of MCP-1 mRNA and associated increase in monocyte chemotactic activity present in culture supernatants of PAMs from injured rat lungs suggest that PAMs may participate in the pathogenesis of acute inflammatory lung injury by the secretion of monocyte chemoattractants including MCP-1.