Defective efferocytosis in the airway may perpetuate inflammation in smokers with/without chronic obstructive pulmonary disease. Mannose-binding lectin (MBL) improves efferocytosis in vitro; however, the effects of in vivo administration are unknown. MBL circulates in complex with MBL-associated serine proteases (MASPs), and efferocytosis involves activation of cytoskeletal-remodeling molecules, including Rac1/2/3. We hypothesized that MBL would improve efferocytosis in vivo, and that possible mechanisms for this effect would include up-regulation of Rac1/2/3 or MASPs. We used a smoking mouse model to investigate the effects of MBL on efferocytosis. MBL (20 microg/20 g mouse) was administered via nebulizer to smoke-exposed mice. In lung tissue (disaggregated) and bronchoalveolar lavage (BAL), we investigated leukocyte counts, apoptosis, and the ability of alveolar and tissue macrophages to phagocytose apoptotic murine epithelial cells. In human studies, flow cytometry, ELISA, and RT-PCR were used to investigate the effects of MBL on efferocytosis, Rac1/2/3, and MASPs. Smoke-exposed mice showed significantly reduced efferocytosis in BAL and tissue. Efferocytosis was significantly improved by MBL (BAL: control, 26.2%; smoke-exposed, 17.66%; MBL + smoke-exposed, 27.8%; tissue: control, 35.9%; smoke-exposed, 21.6%; MBL + smoke-exposed, 34.5%). Leukocyte/macrophage counts were normalized in smoke-exposed mice treated with MBL. In human studies, MBL was reduced in chronic obstructive pulmonary disease and in smokers, and was significantly correlated with reduced efferocytosis ex vivo. MASPs were not detected in BAL, and were not produced by alveolar or tissue macrophages. MBL significantly increased macrophage expression of Rac1/2/3. We provide evidence for Rac1/2/3 involvement in the MBL-mediated improvement in efferocytosis, and a rationale for investigating MBL as a supplement to existing therapies in smoking-related lung inflammation.