Abstract: Surgical implements used during an operating procedure are detected in human or animal tissue. Markers attached to the surgical implements change their impedance at a preselected frequency in the presence of an electromagnetic field. Each of the markers is thereby provided with signal-identifying characteristics. The signal-identifying characteristics are detected by a freely movable, portable detection unit from a first location proximate the patient in an accurate, reliable manner. Potential cross-interference with operating room electronic instrumentation is minimized by shielding. System cost and space requirements are substantially lowered. An accidentally retained surgical implement is discovered in a timely manner, prior to completion of surgery and before the patient has been removed from the operating room.