Carl Icahn issued a warning about the stock market on Thursday.
"I really think even though earnings are going to be very good ... I just think this thing has gotten into a euphoric state," the billionaire activist investor told Scott Wapner on CNBC's "Halftime Report."
But the investor did acknowledge there is a lot of underlying strength to the market. "It's run away and there might be a big correction but I can't say it's insane," he said.
The widely watched Dow Jones industrial average crossed 24,000 for the first time Thursday as corporate America and investors put their hopes on the passage of tax cuts. The broader is up 18 percent this year, and more than 2.6 percent this month.
On Thursday, President Donald Trump again took credit for the market gains, saying in a message on Twitter that had the Democrats won the presidential election the market would be down 50 percent.
Icahn was an advisor to Trump on regulation until August, when he abruptly stepped away from that informal job amid criticism for his role in an energy company that was seen as a conflict.
But he said Thursday he believed that the pro-business agenda embraced by Trump during the campaign would translate into growth. "I really thought Trump was really going to change the landscape, and he certainly has gone far to do that."
The activist also spent some time on Thursday talking about his latest investment. On Thursday he sent a letter to SandRidge Energy blasting its attempts to block him from acquiring more shares. Earlier this month, he disclosed a 13.5 percent stake, making him the energy company's largest shareholder and an outspoken critic of its plan to acquire rival Bonanza Creek Energy.
Another shareholder, Fir Tree Partners, is also opposing the Bonanza Creek deal. Icahn is considering a proxy fight on the proposed acquisition.
In a statement to CNBC, SandRidge said, "The acquisition of Bonanza Creek is highly strategic for SandRidge" and "will create long-term value for shareholders."
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— CNBC's Tom DiChristopher contributed to this report.