A new fluorescence property of yeast D-glyceraldehyde-3-phosphate dehydrogenase was detected after it had been incubated in KI solution. The fluorophore formed in Baker's yeast GAPDH fluoresced at 383 nm by excitation at 336 nm. The formation of the fluorophore depended on NAD+. Modification of Cys-149 with iodoacetic acid influenced some properties of the fluorescence. It appears that the fluorophore may be situated at or near the active site.