Thirty-eight strains of microorganisms isolated from infected human trachea, urine, blood and faeces were examined for their ability to form N-nitrosomorpholine from morpholine and nitrite at pH 7.25. Twenty-five bacterial strains exhibited nitrosation activity, including 18 out of 19 strains of Escherichia coli and three out of nine Pseudomonas aeruginosa, Proteus morganii, Proteus mirabilis, Klebsiella pneumoniae and Neisseria strains; E. coli A10 strain showed the highest activity. Formation of N-nitrosomorpholine was proportional to the incubation time up to 2 h and to the number of resting E. coli A10 cells; the reaction followed Michaelis-Menten kinetics. Nitrosation rate appeared to be dependent on the pKa value of several amines studied. As the nitrosation catalysis was heat-labile, our data suggest that N-nitrosation is catalysed by a bacterial enzyme(s). This reaction may lead to enhanced endogenous nitrosation in subjects suffering from an achlorhydric stomach or from chronic urinary tract infections.