• CNBC's Jim Cramer on Monday offered two airline stocks that he believes investors should pick up for their portfolios.
• "Just remember to ring the register gradually on the way up, because remember, these are airlines. They tend to be a very boom and bust industry," the "Mad Money" host said.

CNBC's Jim Cramer on Monday offered two airline stocks that he believes investors should pick up for their portfolios.

"There's always a bull market somewhere and right now it's flying at 30,000 feet high. My favorites are the two most profitable, that's [Delta Air Lines] and [Alaska Air Group]. Just remember to ring the register gradually on the way up, because remember, these are airlines. They tend to be a very boom and bust industry," the "Mad Money" host said.

Shares of Delta fell 0.96% on Monday while Alaska stock slipped 0.19%.

Delta said earlier this month that it expects unit revenues to increase double digits in the second quarter compared to pre-pandemic, three years ago. The company also expects overall sales to recover up to 97% of 2019 levels

Chief executive Ed Bastian said on "Squawk Box" on the heels of the company's latest quarterly results that the airline recorded its highest ever monthly sales in terms of bookings in March and that this trend is continuing into April. 

"I'm still stunned," Cramer said of Bastian's comments.

Alaska set a sales record in March but trimmed its schedule 2% through the end of June due to a pilot shortage.

"Although they're not one of the majors, it is extremely well-run, still, with a much higher mix of leisure travelers compared to business ones," Cramer said.

"The only problem with this stock is that everybody knows Alaska Air's one of the strongest players in the industry, which makes it harder for them to deliver an upside surprise. That's why the stock is actually down a few bucks from where it was trading before the quarter," he added.

Cramer said that even though there is a bull market in airlines, there are a few companies whose stocks investors should avoid.

"I'd steer clear of the companies involved in the bidding war for Spirit Airlines – that's JetBlue, Frontier and Spirit itself," he said.

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