What is the proper plural form of "Mr."?

sirs, i think.

I concur.

Actually, it's Messrs.

i agree with gavel its Messrs (IPA pronunciation: ['mes%u0259(r)z])
http://en.wikipedia.org/wiki/Mr.

I would probably try to avoid "Messrs." in most contexts. "Sirs" could work; writing Mr. twice seems better most of the time, or writing "gentlemen" and avoiding the plural Mr. altogether.

It depends on whom you are addressing and in what format. Are you writing to lawyers? Sending out invitations? Addressing a group of people? The audience makes all the difference as to which plural is appropriate.

The plural of sir is sirs. The plural of Mr is Messrs. So, Dear Sirs, to a group of men whose names you don't know (or need to use) and Messrs Rolls and Royce built cars for the rich and famous.

This has all been quite helpful/illuminating, thanks. I'm actually writing to a pair of potential employers, in an uber-casual workplace. It would ALMOST be ok to use their first names.

@mirza – ”(IPA pronunciation: [‘mes%u0259®z])” – That must be a bitch to pronounce!