He arches his eyebrow, entirely skeptical of the idea that Tony wouldn't risk himself without having properly tested everything. Maybe Tony thinks he can do it, but there's a difference between Tony knowing he's ready and Tony actually being ready. The idea is pretty terrifying; there's so much that can go wrong, even if he supposedly knows what he's doing, even if the serum works.

"You fixed it once. In one person who isn't you. What if your body reacts to it differently? What if you get something a tiny bit different in the serum this time? This isn't something you should play with, Tony. Not when you're -- " He stops, clenching his jaw, and he lowers his eyes.

"Don't fix what isn't broken, right?"

Because Bruce intends to find out if this serum can cure him, but not before he checks everything first.