Results are presented for the determination by differential potentiometry of calcium and potassium in human and sheep sera, by use of a flow-through cell. The split-membrane ion-selective electrode technique has been used in conjunction with polymer membrane electrodes of the coated-wire type and the type containing an aqueous inner reference solution. The electroactive reagent is di(n-octylphenyl)phosphoric acid for calcium and valinomycin for potassium.