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What do you do?

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I'm assuming you mean MP bet out on the flop rather than raised on the flop?

This is an easy call IMHO. You probably have other outs besides the nut flush draw. With his bet, he is offering you about 13-5. With implied odds, that should be plenty to see another card.

I don't even mind another caller here. Your real decision is what to do if one of the checkers pushes after you cold call. Also, how are you going to play the turn if an Ace or Jack come, because you could still be behind.