One worker was killed and two injured when carbon dioxide leaked at a Samsung Electronics chip plant on Tuesday, the South Korean company said.
The three were found unconscious in the basement of the semiconductor factory in Suwon just south of Seoul, the firm said in a statement.
A man aged 24 was pronounced dead in hospital hours later and two others, aged 26 and 54, remain unconscious, Samsung said.
All were employees of one of Samsung's suppliers and were inspecting the factory's gas-related facilities, it added.
"It is believed that the cause of death is suffocation due to a carbon dioxide leak," Samsung said, adding it was investigating.
Samsung - the world's top maker of smartphones as well as memory chips - operates a vast semiconductor production compound in Suwon.
Fatal accidents at major companies have made frequent headlines in the South, which has struggled to improve poor safety standards in factories.
In January four workers suffocated due to a gas leak at a steel factory owned by Posco - the country's top steelmaker - in the southern city of Pohang.
In August last year four workers died when a fuel tank exploded in a vessel under construction at the headquarters of STX Offshore and Shipbuilding in the southern city of Changwon.