It is found that rutin can react with yttrium(III) (Y 3+ ), and emits fluorescence of rutin. The intensity is greatly enhanced by proteins in the presence of sodium lauryl sulfate (SLS). Based on this, a new fluorimetric method of determination of proteins is developed. Under optimum conditions, the enhanced intensity of fluorescence is in proportion to the concentration of proteins in the range of 5.0×10 −9 –1.0×10 −5 g/mL for bovine serum albumin (BSA), 3.0×10 −8 –1.0×10 −5 g/mL for human serum albumin (HSA) and 1.0×10 −7 –2.0×10 −5 g/mL for egg albumin (EA). Their detection limits (S/N=3) are 1.6×10 −9 , 9.8×10 −9 and 2.1×10 −8 g/mL, respectively. The interaction mechanism is also studied.