Acute lung injury/Acute Respiratory distress syndrome (ALI/ARDS) is an important cause of mortality in critically ill patients. Macrophages play an important role in the pathogenesis of ALI/ARDS. To investigate the role and underlying mechanisms of circulating monocytes and resident alveolar macrophages (AMs) in ALI/ARDS, we depleted circulating monocytes and AMs by clodronate-loaded liposome (CL) in lipopolysaccharide (LPS)-induced ALI/ARDS mouse model. Our results indicated that depletion of circulating monocytes by intravenous (i.v.) injection of CL 2 days prior to intra-tracheal (i.t.) LPS treatment significantly suppressed the acute lung injury in mice with ALI/ARDS, accompanied with significant reduction in neutrophil influx, interleukin-17 (IL-17), monocyte chemoattractant protein 1 (MCP-1), high-mobility group box 1 protein (HMGB1), suppressor of cytokine signaling 3 (SOCS3) and surfactant protein D (SP-D) in the lungs of 2 days LPS-i.t. treated mice. In contrast, depletion of AMs by i.t. delivery of CL enhanced the acute lung injury in association with up-regulation of these mediators. Blocking MCP-1 signaling by intraperitoneal (i.p.) instillation of anti-mouse CCL2 neutralizing antibody significantly reduced acute lung injury and neutrophil influx. In addition, SP-D was up-regulated by mediators released from AMs, because primary murine type II alveolar epithelial cells (AECII) expressed more SP-D after treatment with bronchoalveolar lavage (BAL) from LPS-treated mice or the conditioned media from LPS-treated RAW264.7 cells. The results indicated that circulating monocytes are pro-inflammatory, but AMs have anti-inflammatory functions in the early phase of ALI/ARDS. The study provided a molecular basis for the treatment of ALI/ARDS through modulation of circulating monocytes and AMs.