Twelve out of 14 bacterial strains isolated from patients with urinary infections and nine out of 30 microorganisms isolated from gastric juice from patients with gastric achlorhydria were shown to catalyse the formation of N-nitrosomorpholine (NMOR) from nitrite and morpholine at neutral pH. The effects of various metal ions and cofactors on the bacterial nitrosation reaction was investigated. The presence of nitrate in the culture medium was required to induce nitrosating activity in bacteria, but low nitrate concentrations inhibited the nitrosation reaction.