Music fans who took souvenir T-shirts from a rock festival in Gera, eastern Germany, have discovered they hold a secret message.
The so-called Trojan T-shirts bore a design of a skull and right-wing flags and the words "hardcore rebels".
But, once washed, the design dissolves to reveal a message telling people to break with extremism.
Some 250 of the T-shirts were donated to organisers, who handed them out at the Rock For Germany festival.
The stunt was organised by a left-wing group called Exit, which seeks to reduce the influence of the right-wing in Germany.
"What your T-shirt can do, so can you - we'll help you break with right-wing extremism," the message reads, and provides a contact number for the group.
On its website, Exit said it had made contact with the organisers of the Rock For Germany festival, in its ninth year, using a false name and had offered the T-shirts for free.
The group's founder, Bernd Wagner, said the group hoped its actions would raise awareness among young festival-goers.
"With these T-shirts, we aimed to make ourselves known among right-wingers, especially among young ones who are not yet fully committed to the extreme right," he said, according to the Agence France-Presse news agency.
There was no official reaction from the festival's organisers but a text message was circulating among young people who attended warning them that the T-shirts were fake.