The technique for polymer surface modification described in earlier papers has been applied to two components of an extracorporeal blood circulatory system used in open-heart surgery, with the object of improving their haemocompatibility. The devices were the arterial blood filter composed of nylon membranes and the blood oxygenator consisting of polypropylene microporous hollow fibres. Polyacrylamide and, in some cases, a mixture of polyacrylamide and poly(3-aminopropyl methacrylamide) have been covalently attached to the blood-contacting surfaces. The basic groups of the latter polymers provided a means of coupling heparin to the surface. The extent of heparin coupling was determined by radioactivity measurements with the aid of S-35-labelled heparin. The following determinations were made to assess haemocompatibilities : blood plasma clotting times (PTT); platelet adhesion to the nylon filters; concentrations of lymphocytes and neutrophils in whole blood after exposure to the surfaces. It is concluded that the overall process, which has the merits of cheapness and simplicity in application, has a beneficial effect on haemocompatibility.