N4-Aminocytidine was readily formed on treatment of cytidine with hydrazine-bisulfite under mild conditions. This reaction can be carried out for cytosines in polynucleotids as well. The nucleoside N4-aminocytidine was strongly mutagenic to bacteria and phage. The transformation of cytosines in polynucleotides into N4-aminocytosines is a convenient method for generating reactive groups in the polymer.