Abstract: Targets (30) of readily saturable mangetic material and mounted on protected articles (14) are detected when taken through an interrogation zone (24) in which an alternating magnetic interrogation field is generated by transmitter antenna coils (42, 44). The target (30) is driven alternately into and out of saturation and it disturbs the alternating magnetic interrogation field in a manner so as to produce alternating magnetic fields at frequencies which are harmonics of the frequency of the alternating magnetic interrogation field. The composite of these alternating magnetic fields has a characteristic asymmetry due to the effect of the earth's magnetic field. The target responses are detected by receiver antenna coils (50, 52) to produce first detection signals which are processed in a compressor (118) and a signal averager (124) to produce asymmetry signals which are compared in a comparator (146) with the first detection signals to produce alarm signals.