Here it is worded a bit better:
What is the longest word able to be typed by alternating hands (one letter on left side, one on right, next on left, etc.)?
I got this given to me in a trivia chat room.
Easy: it’s the same as the longest regular word. Just hunt & peck for the first letter with your left hand, then hunt & peck for the second with your right hand, etc.
(Yes, I know what you meant, I’m just a smartass.) 
From A Collection of Word Oddities and Trivia, Page 8
LEPTOTHRICOSIS, from what a gathered from a Google search, is apparently another name for Actinomycosis, a bacterial disease known as “lumpy jaw”. LEUCOCYTOZOANS is the plural form of a parasitic bird protozoa. Both have 14 letters.
[Edited by Alphagene on 10-15-2000 at 12:12 PM]
Writing that perl script to check /usr/dict/words turns out to have been a waste of time.
For the record, though, it cranked out “authenticity”.
Guess I need a bigger dictionary.
You probably knew this, but the longest word that can be typed all on the top row is “typerwriter” :0)
On the link provided above it says:
Byron Davidson believes EIGHTY-SIX is the largest number which when spelled out is typed with alternating hands, ignoring the hyphen.
umm… would it be eighty-eight?
You mean “typewriter”, right? That has 10 letters but so do…
And they are all top-row letters.
Unless, of course, the keyboard is the Dvorak layout
How about when not spelled out?
9494949494949494949494949494949494…
Actually, there are three words with eleven letters matching this definition (depending on your dictionary, I suppose):
proprietory, proterotype, rupturewort
(from the single word spelling section of the rec.puzzles archive.)
the longest word without repeating a letter is uncopywritable
Ajeudicowltivurmapenty
22 letters
Adj:
It’s how you describe a religious bovine showing up for a class on cartography.
So is stewardesses still tops for one handed words?
For fairly common words, sweaterdresses beats stewardesses by two letters.