Disclosed are methods of conjugating biologically active substances, particularly, alpha-interferon, with a hyaluronan or a mixture of a hyaluronan with at least one other hydrophilic polymer having a functional group capable of reacting with divinyl sulfone. Also disclosed are stable intermediates formed by partially reacting a hyaluronan with divinyl sulfone and stopping the reaction before completion to leave free, or reactive vinyl groups on the hyaluronan molecule available for conjugation with the biologically active substance.