The brick is the eighth in a series of 10 created by Gavin Turk
A London gallery is looking for a £3,000 brick by artist Gavin Turk which went missing and was replaced with a worthless equivalent.
Revolting Brick, signed by the artist, was substituted with one from a pile which exhibition-goers were invited to take from after their visit.
"We are treating this seriously - important art has been taken," said Area10 Project Space's Dimitri Launder.
He added that Turk has alerted art valuers and auctioneers.
Organisers noticed that his brick had been substituted shortly before the show was due to end at the Peckham gallery.
Gavin Turk is renowned for his challenging contemporary artworks
The replacement brick had "thank you, have a nice day, next" boldly printed on it, and was from the pile for visitors to take for free if they wished.
Mr Lautner told the BBC that Turk was "flattered and a little disappointed".
The gallery's co-director and founder called the thief a "prankster". They have notified the police and are offering a reward for information.
Turk's involvement in the project attracted around 1,000 visitors to the exhibition.
The 42-year-old artist, a contemporary of Damien Hirst and Tracey Emin, sells work worth thousands of pounds.