Abstract Methylation of guanosine 5′-phosphate under suitable conditions gives 1-methylguanylic acid. This was converted into the pyrophosphate which was then polymerised using polynucleotide phosphorylase from Escherichia coli. Some physical properties of poly N 1 - methylguanylic acid and the formation of a stable organised structure at acid pH are described. Further methylation of the polymer gives poly 1, 7-dimethylguanylic acid. Analogous derivatives of polyinosinic acid have also been studied.