A U.S. Air Force Lockheed Martin F-35 Lightning II gets into position for a midair refueling.

President-elect Donald Trump is at it again. He took to Twitter on Monday morning to criticize the cost of the Defense Department’s most expensive weapons program, the F-35 Joint Strike Fighter.

Trump called the cost of the F-35 program “out of control,” driving jet builder Lockheed Martin’s stock price down 4 percent by midday and causing its market value to dro by $4 billion.

In his tweet, Trump said he plans to save “billions of dollars … on military (and other) purchases” when he takes office next month.

Trump’s Twitter attack on Monday coincided with the delivery of two F-35s to Israel. Lockheed’s head of the project, Jeff Babione, said he’d welcome the “opportunity to address” the president-elect’s questions about the project.

“We're investing hundreds of millions of dollars to reduce the cost of sustaining the airplane over its 30- to 40-year lifespan,” Babione said in a statement. “We understand the importance of affordability, and that's what the F-35 has been about.”

Arizona Sen. John McCain also recently criticized the F-35 program’s high cost.

This is Trump’s second Twitter attack on an aircraft manufacturer this month. Last week, the president-elect lambasted Boeing, similarly calling the cost of its Air Force One program “out of control” and causing its stock price to drop.