Turkish police say they have seized 1.4kg (3lb) of radioactive material during an anti-smuggling operation in the capital, Ankara.
The material, which police said was the element Californium, was found when a car was searched in the Pursaklar suburb of the city, NTV news channel reported (in Turkish).
Four people in the car were detained.
Police said the suspects were part of a group that had planned to sell the material for more than $70m (£49m).
Pictures supplied by the authorities showed vials and a large packet containing a granular mixture laid out on a table.
It was not clear where the seized material had originated from. NTV said it had been taken to the Turkish Atomic Energy Authority (TAEK) for further examination.
Californium is believed to be produced only in the US and Russia. The isotope Californium-252 is used in nuclear reactors, in portable metal detectors and also in medicine to treat some forms of cancer. However, it is produced in very small quantities.
The element was first synthesized by a university laboratory in the US state of California in 1950.