Anaerobically prepared serum samples from healthy subjects were simultaneously analysed for Ca++ in a flow-through-system of three ion selective disc-electrodes (electrochemical multimeasurementsystem). Two of the sensors contained electrically charged ligands, the third contained a neutral carrier. With neutral carriers an improvement of performance was noted in that there was a large increase in selectivity toward H+- and NH+4-ions and a distinct enhancement of reproducibility.