The fluorescence of Calcein Blue (CB) is quenched by the Fe ²⁺ ion in the Calcein Blue-Fe ²⁺ complex. When dopamine is added to a solution of the Calcein Blue-Fe ²⁺ complex, a dopamine-Fe ²⁺ complex is formed as the result of a ligand exchange reaction between Calcein Blue and dopamine which permits the fluorescence from Calcein Blue to be recovered. The fluorescence intensity at the wavelength of 440 nm (at the excitation wavelength of 340 nm) is found to be proportional to the concentration of the dopamine added to the Calcein Blue-Fe ²⁺ complex solution, which permits dopamine to be quantitatively determined. The Calcein Blue-Fe ²⁺ complex shows a high selectivity for dopamine among the catecholamines and related compounds examined.