Police detained three people for trying to sell 1 kilogram (2.2 pounds) of an unspecified radioactive material for US$1 million (680,000 EUR).
The Czech news agency CTK, citing unconfirmed reports, said the material was enriched uranium. Slovak police spokesman Martin Korch declined comment on the report, saying specialists were examining the material.
Two of the suspects were arrested in eastern Slovakia, and the third was arrested in Hungary, Korch said. The suspects were not identified.
Slovak and Hungarian police have been working together on the case for several months, Korch said.
Erich Tomas, a spokesman for the Slovak Interior Ministry, said he had no information about the case. The U.S. Embassy in Bratislava, the Slovak capital, had no immediate comment.
There have been concerns that Eastern Europe could be a source of radioactive material for a so-called "dirty bomb" designed to kill people by dispersing radiation.
In 2003, police in the Czech Republic, which borders Slovakia, arrested two Slovaks in a sting operation in the city of Brno after they allegedly sold undercover officers bars of low-enriched uranium for US$715,000 (485,000 EUR).
Melissa Fleming, a spokeswoman for the Vienna-based International Atomic Energy Agency, said the U.N. nuclear watchdog would be following up on the case.
"It will be important to determine whether the material in question is nuclear," Fleming said. Such incidents are tracked in an IAEA database, she said.
Concerns about nuclear smuggling have generally been focused on Russia and countries of the former Soviet Union, where security at nuclear-related industries deteriorated after the 1992 Soviet collapse.
The U.S.-based Nuclear Threat Initiative, an organization dedicated to reducing the global threat from nuclear weapons, said in a report last year that Russia remains the prime country of concern for contraband nuclear material.
In 2006, Georgian agents working with CIA officials set up a sting that led to the arrest of Russian citizen who tried to sell a small amount of weapons-grade uranium that he had in a plastic bag in his jacket pocket.
In 1997, two men who officials said planned to smuggle 5 kilograms (11 pounds) of enriched uranium to Pakistan or China were arrested in the Siberian city of Novosibirsk. That uranium reportedly had been stolen from a plant in the former Soviet republic of Kazakhstan.