We propose to detect the charge redistribution of an antibody-antigen complex (a precipitate), formed in a layer on top of an ISFET, by means of a.c. measurements. In fact an increased specific conductivity of the precipitate is detected, caused by the amount of counterions, which is related to the fixed protein charge in the precipitate. We observed that an HSA—antiHSA immune complex, deposited onto an ISFET after immersing in an electrolyte, resulted in a cut-off frequency (−3 dB) that was considerably higher than that of a bare ISFET. The cut-off frequency decreased due to outdiffusion of ions until an equilibrium value was reached, but this was still higher than the typical value for the original bare ISFET. It was concluded that in the equilibrium state, the increased conductivity of the protein layer was the result of the counterions, which screen the fixed charges in the precipitate. Ionic strength and pH cause important changes in the effects. The measurement of the cut-off frequency may be used to determine the isoelectric point of a protein complex.