The protein composition of the cytoplasmic membranes of Escherichia coli, grown aerobically and anaerobically on a glucose minimal medium at pH 7.0, were analyzed by crossed immunoelectrophoresis. Qualitative differences are limited to only two proteins: nitrate reductase (E.C. 1.7.99.4) is absent under aerobic growth conditions, whereas an unidentified protein, with a molecular weight of 81,500 and located at the inner side of the cytoplasmic membrane, is synthesized only in the presence of oxygen. Quantitative differences are observed for many proteins: the ratio of the amount of a specific protein present in cells grown anaerobically and aerobically was, for four proteins, between 0.3 and 1; for 25 proteins, between 1 and 3; and for five proteins, larger than 5.