Access control equipment includes an interrogation unit which emits an interrogation signal, and a plurality of transponders, each transponder having a stored identity code different from that of the other transponders. This identity code includes a plurality of fields each holding a selected information bit. The interrogation signal is controlled so as simultaneously to interrogate the fields of all transponders within range in a serial manner. A group reply signal is sent back to the interrogation unit from any transponder having, in the field being interrogated, a bit matching that required by the interrogation signal. The interrogation unit is arranged to determine, from the series of received reply signals, the identity of each and every valid transponder within range.