French police found 15 illegal Bulgarian immigrants, who had been sheltered at an abandoned industrial building in the town of Chambery.
The Bulgarians have been hired as construction workers.
The raid was part of a large-scale police operation against an illegal organization for traffic in humans in South-eastern France, made up of a Bulgarian and Turkish network.
Twenty-one immigrants have been arrested in the operation.
A Turkish national, one of the heads of the organization, has been charged with traffic in humans.
The Bulgarians, who have been hired as construction workers, lived in extremely poor conditions, AFP reported.