President-elect Donald Trump warned U.S. companies that moving manufacturing to other countries would be a “very expensive mistake” as they will face heavy new taxes.

In an early-morning series of six Twitter posts, Trump summarized a plan that was long part of his campaign position: that shifting production from the U.S. to Mexico, China or other lower-cost countries would be discouraged by tax policy.

“Any business that leaves our country for another country, fires its employees, builds a new factory or plant in the other country, and then thinks it will sell its product back into the U.S. without retribution or consequence, is WRONG!” Trump said. “There will be a tax on our soon to be strong border of 35 percent for these companies wanting to sell their product, cars, A.C. units etc., back across the border.”

Trump also said he plans to “substantially” reduce taxes and regulations on business.

“Please be forewarned prior to making a very expensive mistake,” Trump said. He concluded, in capital letters, that “the United States is open for business.”

Trump this week reached a deal with United Technologies Corp. to keep about 1,100 jobs at a Carrier air-conditioning plant in Indianapolis rather than move them to Mexico. The president-elect signaled he will frequently deal with corporate leaders to protect American workers.

“We’re going to have a lot of phone calls to companies that say they’re thinking about leaving this country, because they’re not leaving this country,” Trump said on Dec. 1 at the Carrier plant in Indiana.