The absorption, fluorescence emission, fluorescence excitation and fluorescence polarization spectra of polyriboguanylic acid (poly(G)) were studied in aqueous solution at room temperature. Greatly red-shifted fluorescence from poly(G) in water was attributed to a small amount of the bases protonated on the N(7) atom. This was supported by the following points: (1) the fluorescence emission spectrum of guanosine 5'-monophosphate in acid pH conditions was very close to that of poly(G) in water, and their fluorescence excitation spectra were coincident, and (2) the red-shifted fluorescence band disappeared on increasing pH with the addition of tris(hydroxymethyl) amino methane. The fluorescence titration curve exhibited a maximum at pH 4.8 that was assigned to the four-stranded form of poly(G), with part of the bases being protonated. The protonation degree was found to depend on the ionic strength of the solution.