4-Hydroxy-6-(l-erythro-1,2-dihydroxypropyl)-2-(methylthio)pteridine was converted by ethylenediamine to 2-(2-aminoethylamino)analogue of biopterin, which condensed with N-hydroxysuccimidyloxy m-maleimidobenzoate to give the maleimidobenzamide. The latter compound and β-d-galactosidase afforded a biopterin-galactosidase conjugate which showed the enzyme activity about 40% as compared to the unmodified enzyme.