A Victorian painting, which has been hanging undiscovered in a New England beach house for 50 years, could fetch £500,000 at auction.The early version of William Powell Frith's The Derby Day goes on sale at Christie's in London next month.
The finished painting now hangs in the Tate Britain gallery.
A friend of the owner suggested the oil painting of an 1850s horse race may be worth something. Christie's Peter Brown said the find was "immensely exciting".
Brown said the anonymous vendor, now in his 60s, believes his parents bought the painting sometime before World War II.
At that time, Victorian art was not well regarded by art critics.
"It's a testament to the change in fortunes of Victorian pictures over the last century that these things could have been acquired very cheaply indeed in the 30s, 40s and 50s," said Brown, Christie's director of Victorian pictures.
The final version of the The Derby Day (1856-1858) proved so popular when it was first exhibited at the Royal Academy in 1858, that a rail had to be put up to keep back the crowds and a policeman had to guard it.
The action-packed painting presents a satirical panorama of modern Victorian life.
Frith's other famous works include Ramsgate Sands and The Railway Station.
Other works to be auctioned on 15 December include Frank Cadogan Cowper's Our Lady of the Fruits of the Earth and two portraits by Sir John Everett Millais.
Dante Gabriel Rossetti's Portrait of Annie Miller is also on offer. (BBC News)