The ion-step method was originally designed to measure immunoreactions via the change in charge density which occurs in an antibody loaded membrane, deposited on an ISFET, upon reaction with a charged antigen. In this paper we show the possibility of detecting an immunoreaction involving a non-charged antigen, progesterone, by using competitive binding of progesterone and a charged progesterone-lysozyme conjugate in the membrane. Both the immunoreaction and the non-specific binding could unequivocally be measured. The detection limit was approximately 10−8 M of progesterone in the sample solution. This proves that the ion-step method is suitable for detection of both charged and non-charged antigens.