The action of O-methylhydroxylamine in vitro upon some amber and rII ochre mutants of T4 phage leads to a considerable increase in the frequency of reversions and conversions of ochre and opal mutants presumably due to A yields G transitions. The transitions seem to be caused by modification of the adenine bases of the genome to give N-6-methoxyadenine which has equivocal (A- and G-like) specificity. However, the functional activity of N-6-methoxyadenosine residues in the RNA polymerase system is low. We failed to detect any significant activity of these residues either as components of a template polyribonucleotide, or as nucleoside triphosphate precursors.