The derivatization of bovine serum albumin with fluorescamine was examined utilizing the tube radial distribution phenomenon of the ternary mixed solution of water-acetonitrile-ethyl acetate (3:15:8 volume ratio) in a fused-silica capillary tube. Double capillary tubes having different inner diameters (100 and 250 mu m i.d.) were designed to promote the tube radial distribution phenomenon. The smaller tube was inserted into the larger one through a T-type joint. A water-acetonitrile mixture (3:1 volume ratio) including bovine serum albumin was delivered into the large tube from the inside through the small tube, and an acetonitrile-ethyl acetate mixture (7:4 volume ratio) including fluorescamine was delivered from the outside through the joint. The solutions were mixed through the large tube to perform the derivatization reaction. The fluorescence intensity of the fluorescamine-derivatized bovine serum albumin obtained under the tube radial distribution phenomenon was greater than that obtained through a batch-reaction using a homogeneous solution of water-acetonitrile (1:5 volume ratio).