Paulie Malignaggi slapped Artem Lobov as the pair's bare knuckle boxing rivalry reached boiling point.
The pair have both signed with Bare Knuckle Fighting Championship and appear on a collision course for a summer showdown.
Lobov must first defeat Jason Knight on Saturday - but the seeds were sown at a media day on Tuesday.
As Malignaggi and Lobov - a former teammate of Conor McGregor - faced off, the former world champion boxer slapped the former UFC star.
He then vowed to send him home in a body bag should the two clash in the ring.
Malignaggi said: "Don't threaten me here. The next time it'll be a real slap; that was a b**** slap for a b****.
"Don't get in my face and make threats here cos I'll f*** you up; next time I'll smack you up for real.
"You're on a death wise bro; I'm going to put you in a body bag in June... I'm going to kill you."
Lobov, who won two of his seven fights in the UFC, said: "I'm a real fighter and I'm going to fight Paulie in a real fight. I want to make money for it. He knows I'm down. If you're a real fighter, let's fight in the ring.
"Let's be honest, Paulie doesn't want to fight me on the street, because he knows I'd tear him apart in seconds. But I want to prove myself in the boxing ring. That's why I'm doing this. I want Paulie safe until the day I can finally put my hands on him.
"This is the only chance he has to get a shot in on me. When we do get to fight, he won't get a hand on me, I guarantee that.
"I'm here to fight on Saturday night. I'm a true fighter. No one will ever make me quit. I don't care about anything that Paulie can do to me.
"Paulie was always the fight I wanted. It's 'night night' for Jason Knight this Saturday, then it's on to Paulie. That's the only fight I want. He deserves to get slapped around."
