Abstract This communication characterizes the excitation spectra of enhanced Tb 3+ emission at 545 nm due to energy transfer from nucleic acid bases. For 5′GMP and polyguanylic acid the excitation spectrum maximum occurs at 252 nm and, for polycytidylic acid, at 270 nm. Both in their maxima and general shape the excitation spectra correspond closely to the absorption spectra of the aromatic chromophores. To obtain reliable excitation spectra it is necessary both to have solutions of low absorbance in order to avoid inner-filtering of incident radiation and to correct for the severe decrease of xenon lamp intensity at < 270 nm. In samples of yeast and E. coli t-RNA, guanine bases are primarily responsible for enhanced Tb 3+ emission. The two icosahedral viruses, bromegrass mosaic virus and southern bean mosaic virus, display excitation spectra for Tb 3+ emission similar to that of the t-RNA samples. In the viruses, energy transfer to Tb 3+ in a presumed Ca 2+ site originates primarily from guanine bases and not from aromatic chromophores in the protein component.