The present invention is directed to a multistep process for the physical depolymerization of heparin wherein heparin is subjected to at least two .gamma.-ray irradiations and wherein between two irradiation steps the depolymerised heparin is subjected to a separation step and only a fraction of the depolymerised heparin obtained from the first irradiation is subjected to the second irradiation step. It is also directed to heparin-derived oligosaccharide fractions obtainable by the process of the invention.