Pure honesty from Larry Holmes ... who says his decades-old feud with Leon Spinks was real and mean, but he's praying for his old rival to fight through his current medical crisis.

If you know your boxing history ... you know Holmes had REAL animosity for both Leon and his brother Michael Spinks.

In fact, Holmes knocked out Leon in the 3rd round of their 1981 title fight -- only to get beat TWICE by Leon's bro Michael later in his career.

Now, with Leon fighting for his life in a Las Vegas hospital, we asked Holmes how he felt about the situation ... and his response was about as real as it gets.

"Leon was a fighter that ... if you ain't ready, don't get in the ring with him! Because he's ready. He fought hard, man."

Here's where things got real ...

"I didn't really like him as much as a fighter because he said some things to my wife that I didn't like," Holmes said ... "And, I held it back on him a little bit."

"But, when it came down to me fighting him, he was total different guy."

Holmes added, "You don't want to kick a man while he's down. That's what a lot of people want you to do, kill him while he's down. I don't want to do that. I'm not mercenary. I got in the boxing game to make money, to take care of my family. Not to hurt nobody."

"And, I hope that I didn't hurt him, but I probably was one of the guys that caused whatever problem that he's having now."