Water Quality in Cell Culture

[Owl]

In commercial antibody plants (large-scale mammalian cell 
culture) the water is certified WFI (water for injection).  For 
lab scale, I'd recommend you stay with 18 megohm water (Milli-Q 
or equivalent) unless you know for certain that your cells don't 
mind metal ions.  Copper, zinc and iron are all potential growth 
inhibitors.
Of course, if you don't have ultrapure water, use what you have, 
and see if it works.  (I've had some bad experiences doing that, 
though.)