Abstract: The invention describes methods for locating a treatment device disposed within a living body by means of magnetic fields that are produced by Barkhausen jumps, principally from amorphous tag wires with high permeability that exhibit reentrant flux reversal. When wires of this type are attached to concealed treatment devices such as catheters, interrogation or scanning of the tag wire by a low frequency ac magnetic field affords an accurate means for locating the treatment devices using a sensor coil to detect the magnetic field signal from the wire locating tag. The strength of the field detected by the position of a sensor coil with respect to the locator tag is used to determine the location of the tag. A favorable signal to noise detection ration is obtained as the signal emitted by the wire is at a very high frequency compared to that of the frequency of the interrogation field.