The ion-step measuring method was used to determine heparin concentrations in PBS and blood plasma. Heparin is a sulphated polysaccharide which is clinically used as a drug to prevent the clotting of blood. The measuring method is based on detection of changes in charge density in a porous membrane which is deposited on the gate of an ISFET. Protamine was used as affinity ligand in the membrane. In PBS a linear relation was found between the heparin concentration and the ISFET response. The incubation time was reduced from 18 h to 15 min by increasing the porosity of the membrane. The results of the measurements in blood plasma show a significant nonspecific binding of plasma components in the membrane. Suggestions are given to prevent this nonspecific adsorption. The results described in this paper show a detection limit for the ion-step measuring method of at least 5 ± 10-11 Mol/l which is promising for future practical applications.