The binding of bromphenol blue (BPB)* to bovine serum albumin (BSA), human serum albumin, or ovalbumin at pH 7.0 caused an enhancement of fluorescence of the dye. The order of enhanced fluorescence intensity of sulfonphthalein dyes bound to BSA was: BPB, brom-chlorphenol blue and bromcresol purple> chlorphenol red> cresol red and bromphenol red> phenol red, bromcresol green, and bromthymol blue. The latter three exhibited little or no enhancement of fluorescence. The fluorescence enhancement is considered to be associated with a planar conformation of dyes induced by their binding to proteins. The intensity of fluorescence of BSA-bound BPB was found to vary with pH. It increased in both acid and alkaline pH ranges (3.2 to 4.5 and 7.0 to 9.0), where conformational changes of the BSA molecule are known to occur. The modification of absorption spectrum corresponding to the fluorescence change was also observed. Below pH 3.2, the intensity of fluorescence decreased accompanied by a spectral change of BPB, which indicated the conversion of the dye from the basic form to the acidic one.