The so-called ion-step method represents a newly developed measurement concept for potentiometric detection and quantification of adsorbed biomolecules in which modified ion-sensitive field-effect transistors (ISFETs) are used. The ion-step method is based on a dynamic measuring principle, whereas many other potentiometric methods are static and measure in a state of thermo-dynamic equilibrium. A number of authors report on measuring protein adsorption by equilibrium potentiometry, but in all measurements the observed responses were poor (1, 2). The new measuring method is therefore an important alternative method for detection of adsorbed protein. In Subheading 1.2., a short explanation will be given for the disappointing results obtained with detection of protein by equilibrium potentiometry. Thereafter, the ion-step method will be described briefly and at the end of this introduction, the use of an ISFET affinity sensor for the measurement of heparin will be described.