Dissidents in communist East Germany were given radioactive "tags" that allowed them to be tracked with Geiger counters.
Files from the German Democratic Republic's secret police, the Stasi, showed that people suspected of being enemies of the state were sprayed with radioactive chemicals.
They could then be followed by agents wearing concealed Geiger counters that triggered a "silent ringer" vibrating alarm. A number of prominent dissidents have died from cancer which could be linked to large doses of radiation.
Evidence of radioactive tracking was found in the vast Stasi archives by officials of the Berlin-based Gauk Commission, a government agency investigating the former secret police.
New Scientist magazine reported that, according to the Stasi files, people were labelled with radioactive substances in a number of ways.
If they could not be sprayed with radioactive chemicals, their cars, documents or paper money would be tagged. The Stasi even developed an airgun that could fire radio-labelled silver wire into a car tyre from 25 metres away.
Barrie Lambert, a radiobiologist at St Bartholomew's Hospital in London, said: "It really is the stuff of James Bond movies. It's an unpleasant thing to do. The risk is not limited to the person being tagged. You'd be exposing other people, such as the spouse."
The radioactive tagging activities of the Stasi have been investigated by radiation protection expert Klaus Becker. He said while doses were usually below what would seriously harm or kill, there were mishaps.
For instance, the Stasi marked West German Deutschmarks with large amounts of scandium, a radiation emitter, to see how and to whom they circulated.
The notes were never retrieved and disappeared without trace. Later, the Stasi calculated that if more than one note was in a man's pocket, the effect on his fertility would be "close to castration".