Alpha-synuclein was originally identified as the presynaptic nerve terminal protein. Recently, we reported that alpha-synuclein is also expressed in cultured human astrocytes and that its levels are increased by stimulation with interleukin-1beta, suggesting that it may be involved in inflammatory processes. We therefore investigated the effect of inflammatory stimuli on alpha-synuclein expression in human macrophages. Alpha-synuclein mRNA and protein were detected in cultured human macrophages and levels of alpha-synuclein protein were increased by stimulation with lipopolysaccharide and interleukin-1beta in a time- and concentration-dependent manner. Immunofluorescent staining showed that alpha-synuclein protein was expressed within the cytoplasm and nucleus. Furthermore, alpha-synuclein immunoreactivity was present in alveolar macrophages from human lung tissues. These findings suggest that the function of alpha-synuclein is not exclusive to the nervous system and that alpha-synuclein may play a role in inflammatory processes and immune responses.