Abstract: New heparin derivatives having antithrombotic activity, also endowed with reduced hemorrhagic and anticoagulant activity, obtained by treating in a basic medium heparins of various origin, optionally in the presence of alkali metal salts and of a reducing agent.The heparin derivatives obtained through this treatment show peculiar chimico-physical characteristics, like new signals at about 53 and 54 p.p.m. in the .sup.13 C-NMR spectrum and an increase of the specific rotatory power, compared to that of the starting heparins, to values between +50.degree. to +90.degree..Said structural modifications produce an improvement of the biological properties of the heparin, substantially keeping the antithrombotic activity while diminishing the hemorrhagic effect in vivo and the anticoagulant activity in vitro.