I'm going to tell you something that's not very healthy to know.
5 seconds sound clip from the The 39 Steps (1937) classic radio play.
You can hear this line at 00:09:41 in the radio play.
Quote context
[...]
- All right. I'll play your game a bit further.
- Oh, don't let them see you! Well?
- By jove, you're right! Two of them. They seem to be looking up here.
- Come away, please. Do you believe me now?
- I'm going to tell you something that's not very healthy to know.
- But now that they've followed me here, you're in it as much as I am.
- Have you ever heard of... the Thirty-Nine Steps?
- Thirty-Nine Steps? No. What are they?
- Those men are acting quickly. You don't know how clever their chief is. Clever, and ruthless.
[...]