A compact and easy-to-operate device for the detection of proteins in solution is described. The device consists of an ISFET pH-sensor in the middle of a Ag/AgCl electrode, on top of which a microporous composite membrane is deposited. The measurement proceeds by offering an ion step and measuring the transient change of the ISFET potential versus the Ag/AgCl electrode. The potential transient is modulated by adsorption of protein to the membrane by incubation in a protein-containing solution. Theory is developed to describe the ISFET potential transient versus the Ag/AgCl electrode starting from the ISFET and Ag/AgCl potential transients versus an external reference electrode. Conditions are formulated under which the Ag/AgCl electrode potential is sufficiently stable to be used as the reference. Both theory and preliminary measurements show that this condition is met under several characteristic measuring conditions. When the internal reference electrode is used, the ion step can be offered simply by manual transference (`dipping¿) of the device between vessels of different salt concentration instead of by using a complex flow-through set-up. Measurements show a remarkable absence of noise in the potential signal of the dipstick device.