The Anti-Defamation League called on Donald Trump's eldest son to apologize Thursday for making a Holocaust reference during a radio interview.

Appearing on a Philadelphia radio station, Donald Trump Jr. accused the Democratic National Committee of rigging the party's primary system with the media's tacit approval.

"They’ve let [Hillary Clinton] slide on every indiscrepancy, on every lie, on every DNC game trying to get Bernie Sanders out of this thing," Trump Jr. said. "If Republicans were doing that, they’d be warming up the gas chamber right now."

The ADL, an international anti-Semitism organization, tweeted Thursday that "trivialization of the Holocaust and gas chamber is NEVER ok."

There has been no immediate response from the Trump campaign.

The Associated Press contributed to this report.