Tubular gel reactors containing Nitrosomonas europaea and Paracoccus denitrificans, which remove nitrogen from solutions through a process of nitrification and denitrification, require oxygen for ammonia oxidation, the first and rate-limiting step in the process. To accelerate ammonia oxidation, high concentrations of oxygen were applied to the reactors instead of air. Although a 50% O-2 : N-2 gas mixture and pure oxygen were both toxic to free N. europaea cells, they actually accelerated ammonia oxidation by N, europaea immobilized within the tubular gel. Indeed, the rate of ammonia oxidation by a tube exposed to pure oxygen was twice that of one exposed to 20% O-2 When the distribution of N. europaea cells within the tubes was investigated using a fluorescently-labeled antibody, colonies were found on the external surface of the tube exposed to 20% O-2, but were located at a depth of 120-300 mum from the external surface in the case of the tube exposed to pure oxygen. The region between the external surface of the gel and the colonies apparently acted as a barrier, reducing the diffusion of oxygen and thus protecting the cells from oxygen cytotoxicity.