Protestors marched through Seoul this week to protest about the ban change
A strike by tens of thousands of South Korean workers over the import of US beef will hit car production, according to the country's biggest carmakers.
Hyundai said the two-hour walk out would cost 2,000 vehicles in lost output, and Kia said the stoppage would affect the production of 900 cars.
The import of US beef had been banned since 2003 after a case of BSE, or mad cow disease, was identified.
The lifting of the ban has caused widespread protests in South Korea.
Protesters argued that the move failed to protect their health - although both governments said the beef was safe.
An estimated 20,000 Kia workers stopped work for two hours and 45,000 Hyundai workers are expected to so the same later.
Hyundai said the disruption would cost it 300bn won ($288m: £145m). The financial cost at its affiliate Kia is expected to be 12bn won.
Analysts suggested that the walkout had affected Hyundai's shares.
"These constant labour disputes and disruptions to production are factored into Hyundai Motor shares, discounting their value," said Choi Dae-shik, an analyst at CJ Investment & Securities.
The South Korean government said that it would "deal sternly with the illegal, politically motivated strike".
However, the Korean Federation of Trade Unions (KCTU) which organised Wednesday's stoppage said it had the right to strike.
"The government calls it illegal but we don't agree with it," said KCTU president Lee Suk-Haeng.
"We have every right to protect our health," he added.
KCTU is also planning to urge consumers to boycott US beef.