Two-dimensional gel electrophoresis (2-DE) with immobilized pH gradients was carried out on total cell lysates and membrane fractions of Paracoccus denitrificans with the aim to characterize differences in protein expression during growth under aerobic and various anaerobic conditions (with nitrate, nitrite or nitrous oxide). Comparative image analysis of the protein pattern revealed several subgroups of the total 800 protein spots resolved that were characteristically induced or repressed in response to individual electron acceptors. The respiratory inhibitor azide also exerted a profound influence upon cellular protein composition. However, since most of the proteins showing an altered expression pattern in cells growing on oxygen differed from those in cells growing on nitrite, we suppose that azide acts mainly indirectly, possibly by influencing other cellular signals. Limited information on the P. denitrificans genome has precluded the identification of more than eight protein spots as yet. A public accessible P. denitrificans 2-DE protein database is currently built up at http://www.mpiib-berlin.mpg.de/2D-PAGE.