Mortierella parvispora and an unidentified bacterium converted trimethylamine to dimethylamine, and the bacterium (but not the fungus) formed dimethylnitrosamine in the presence of nitrite. Dimethylnitrosamine also appeared in cell suspensions of Escherichia coli and Streptococcus epidermidis and in hyphal mats of Aspergillus oryzae incubated with dimethylamine and nitrate. Suspensions of a number of microorganisms produced N-nitrosodiphenylamine from diphenylamine and nitrite at pH 7.5, and soluble enzymes catalyzing the N-nitrosation of diphenylamine were obtained from two of these organisms. In the presence of these enzymes, several dialkylamines were converted to the corresponding N-nitroso compounds.