As the Nasdaq goes for its ninth-straight session of gains on Wednesday, closely followed investor Dennis Gartman told CNBC he's getting worried..
"I'm a little concerned again because you bumped up again in Nasdaq … to overhead resistance," the publisher of The Gartman Letter said in a "Squawk Box" interview. "It's probably not a bad idea to become a little less aggressive on the long side and a little more interested in taking a little protection."
If the Nasdaq finishes higher Wednesday, it would be the index's longest winning streak since September 2010. While the Dow Jones Industrial Average and the were still lower for 2014 as of Tuesday's close, the Nasdaq was up 2.3 percent this year.
Despite his mild concern, Gartman said it's still a bull market for stocks. "The trend is clearly from the lower left to the upper right."
"One does not get short [here]," he added.
In an earlier appearance on "Squawk Box Asia," Gartman said a rush of mergers and acquisitions this year is a sign that corporate America has once again become swamped with expendable cash. He added that this trend is indicative of high valuations in stock markets.
U.S. stocks finished mostly higher on Tuesday as investors cheered a large acquisition, with generic drugmaker Actavis confirming it would buy Forest Laboratories for about $25 billion in cash and stock.
Meanwhile, King Digital Entertainment, the gaming firm behind the wildly popular online game Candy Crush, said Tuesday it would list on the New York Stock Exchange and shares of Tesla Motors climbed to a record after a report that Apple might be interested in buying the electric car maker.
With the U.S. markets alive with activity, Gartman told CNBC that it was a very good signal for the country's economy. "I think it is an example of corporate America that is flush with cash and we have not seen the balance sheets of the United States corporations this flush with cash in my lifetime."
"I've been around for six decades now. So I think it's an example of finally putting that corporate cash to work. I bet you see a great good deal more," he continued. "I think this is the start of a major move."
According to a report Tuesday from research firm Dealogic, the volume of U.S.-focused mergers and aquisitiions stands at $266 billion this year, with 1,183 deals to date. This is the highest volume since the same period in 2000, which saw $278.1 billion in M&A by this point in the year.
Year-to-date, M&A activity in the U.S. is up 12 percent from the $234 billion seen in the same period in 2013, it said, making up 62 percent of total global M&A, the highest year-to-date share on record.
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While some CEOs might think valuations are ideal to strike deals, Gartman warned that it is most likely the opposite.
"It can go on for several months but it's not indicative of lows in corporate stock prices. It is usually indicative of relatively high levels," he said, adding that, historically, CEOs act at "precisely the wrong time."