A mixture of 5- and 6-carboxyfluorescein was activated with1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride in the presence of eitherN-hydroxysuccinimide or pentafluorophenol togivethe corresponding succinimidyl and pentafluorophenyl esters. Theregioisomeric mixtures wereseparated to give the 5- and 6- succinimidyl and pentafluorophenylactive esters in >98% purity.