ABSTRACT As it is known, quantum inductive circuits with charge discreteness show Bloch-like oscillations in electrical current under a dc external voltage. In this paper, the effect of a superimposed ac voltage in the circuit is considered. The Shapiro effect is found to be related to the existence of resonance. Surprisingly, in the limit of low frequency (no resonance), the electrical averaged current exists and has always the same sign. Eventually this allows for an experimental method to measure discrete charge effect in quantum mesoscopic circuits.