will preferably be a polymeric aliphatic amine especially polyethylene imine having a high average molecular weight but any polyamine having a high average molecular weight and carrying free primary amino groups may be used the aminated surface may optionally be further stabilized by reduction with a suitable reducing agent such as sodium cyanoborohydride the modified surface according to present invention has free primary amino groups by which chemical entities may be bound either ionically or covalently especially aldehyde containing chemical entities may be bound by formation of schiff 39 s bases eventually followed by a stabilization reaction such as a reduction to convert the schiff 39 s bases to secondary amines a higher density of reactive amino functions resulting in a higher biological activity when biologically active material is attached the increased biological activity will appear from the two comparative examples showing the density of binding sites as will appear as the degree of heparinization achieved after the surface modification according to present method and according to the prior art as represented by the working example of u s pat no 4 565 740 golander and larsson a comparing the results achieved when working with according to present invention low concentrations of cross linking agent and b comparing the results achieved by these two methods when working with the higher concentrations according to the prior art of crosslinking agent in these examples tubings of polyethylene pe were first etched with potassium permanganate in conc sulfuric acid whereafter the tubings were contacted with the following solutions 1a 1 ml 5 polymin sn and 340 u03bcl crotonaldehyde in 1 1 0 17m borate buffer of ph 9 2a 1 ml 5 polymin sn and.