Effect of both structure of substituents and degree of substitution of polyions derived from poly-vinyl alcohol on water-absorbility and antithrombogenecity of formed polyionic complexes (PIC)have been investigated. It was found that the water content of the PIC formed from p-formylben-zenesulfonic acid substituted PVA was higher than that of the PIC formed from p-formylbenzoicacid substituted PVA. When the substitution degree of polycation increased from 27% to 64%,the water content of the formed PIC increased from 170% to 1270%. Effect of water content andnet charge of PVA-type PIC on the antithrombogenecity was also investigated by plasma recalci-fication method. Effect of both structure of substituents and degree of substitution of polyions derived from poly-vinyl alcohol on water-absorbility and antithrombogenecity of formed polyionic complexes (PIC) have been investigated. It was found that the water content of the PIC formed from p-formylben -zenesulfonic acid substituted PVA was higher than that of the PIC formed from p-formylbenzoicacid substituted PVA. When the substitution degree of polycation increased from 27% to 64%, the water content of the formed PIC increased from 170% to 1270%. Effect of water content and net charge of PVA-type PIC on the antithrombogenecity was also investigated by plasma recalcification method.