It is shown why the current practice of expressing heparin activity in International Units is not an adequate way to deal with the situation that arises after injection of low molecular weight heparins. An alternative approach is proposed. It is taken into account that the fraction of a low molecular heparin that binds with high affinity to antithrombin III contains two fundamentally different components: the material above the critical chain-length of 17 sugar units that has both anti-factor Xa activity and antithrombin activity (ACLM) and the material below that length with anti-factor Xa activity only (BCLM). It is shown how the levels of ACLM and BCLM in a plasma sample can be determined and expressed in micrograms/ml.