The recognition of bacterial lipopolysaccharide (LPS) is principally mediated by either membrane-bound or soluble form of the glycoprotein CD14 and CD14-associated signal transducer, toll-like receptor 4 (TLR4). Recent findings indicate that the serine protease inhibitor, α1-antitrypsin (AAT), may not only afford protection against proteolytic injury, but may also neutralize microbial activities and affect regulation of innate immunity. We postulated that AAT affects monocyte responses to LPS by regulating CD14 expression and soluble CD14 release. Here we show that a short-term (up to 2h) monocyte exposure to AAT alone or in combination with LPS leads to a remarkable induction of CD14 levels. In parallel, a short-term (2h) cell exposure to AAT/LPS significantly enhances LPS-induced NFκB (p50 and p65) activation in conjunction with increased TNFα, IL-1β and IL-8 release. In contrast, longer term incubation (18h) of monocytes with combined AAT/LPS results in a significant reduction in expression of both CD14 and TLR4, inhibition of LPS-induced TNFα, IL-1β and IL-8 mRNA and protein expression. These findings provide evidence that AAT is an important regulator of CD14 expression and release in monocytes and suggest that AAT may be involved in LPS neutralization and prevention of over-activation of monocytes in vivo.