Objective and design Previous in vitro experiments demonstrated that acute-phase protein, alpha 1-antitrypsin (AAT), could act either as an enhancer or as a suppressor of lipopolysaccharide (LPS)-induced cell activation depending on treatment time. Here we investigate how AAT regulates inflammatory responses in the short term when administrated post LPS challenge. Methods Similar experimental setup was used both in vitro and in vivo: human monocytes and neutrophils were stimulated with LPS for 2 h followed by AAT for a total time of 4 h, and C57BL/6 mice were treated intranasally with LPS and 2 h later with AAT and sacrificed after 4 h. Bronchial lavage (BAL) and lung homogenates were analyzed using bio-plex cytokine assay. BAL cell counts were assessed. Results Within 4 h, AAT enhanced LPS-induced tumor necrosis factor-alpha (TNFa), interleukin (IL)-6, and IL-8 release from monocytes and neutrophils. Mice challenged for 4 h with LPS followed by AAT at 2 h showed nochanges in BAL cell counts and higher levels of almost all measured cytokines, specifically RANTES in BAL and IL-12, IL-13, granulocyte colony-stimulating factor (G-CSF), granulocyte macrophage colony-stimulating factor (GMCSF), and IL-10 levels in lung homogenates, than in mice treated with LPS only. Conclusion Within the short term, AAT enhances the magnitude of LPS-induced specific cytokine/chemokine production,which may play an important role inamplification and resolution of acute-phase inflammatory reactions in vivo. © Springer Basel AG 2010.