The present invention relates to conjugates of a polypeptide and an oligosaccharide, wherein the polypeptide is conjugated to at least one oligosaccharide-spacer residue, the oligosaccharide being a synthetic sulfated oligosaccharide comprising 4-18 monosaccharide units and per se having affinity to antithrombin III and the spacer being a bond or an essentially pharmacologically inactive flexible linking residue, or a pharmaceutically acceptable salt thereof. The conjugates of the invention have improved pharmacokinetic properties when compared to the original polypeptides (i.e. the corresponding non-conjugated polypeptides per se).