The phagocytic NADPH oxidase consists of the core proteins NOX2 and p22phox and the regulatory proteins p47phox, p67phox and Rac1. Recently, homologues to NOX2 were identified. While the presence of NOX1 and NOX4 proteins in vascular cells has been shown, their expression levels in other tissues is less clear. We thus examined the protein levels of NOX1, NOX2, NOX4, p22phox, Rac1 and p47phox in colon, pancreas, kidney, liver, lung and stomach using immunohistochemistry. All tissues expressed high levels of NOX1, especially in the epithelia and in macrophages, and moderate levels of NOX4. Low levels of NOX2 were present in all tissues except of alveolar epithelia and the epithelia of the gastric pars pylorica. Rac1 expression was detected in colonic and gastric stroma, in the gastric, renal and broncheal epithelia and in hepatocytes. In contrast, p47phox was only present in the pancreatic and gastric stroma and in alveolar macrophages. Interestingly, p22phox was expressed at very low levels and was not detectable in liver and the alveolar epithelia. These data show that in contrast to p22phox, NOX1 and NOX4 and to a lesser extent NOX2 are constitutively expressed in various organs. Since the interaction of NOX1, NOX2 and NOX4 with p22phox is essential for the function of NADPH oxidases these findings suggest that p22phox is rate limiting in human tissues.