Do I need to count cells for PI staining? - (Nov/13/2008 )

Hi all,
I typically have about 40 samples to analyze for cell cycle profile by PI staining. Is it imperative to count the cells prior to FACSing them? I do know that there are less cells in my drug treated plates compared to vechicle alone treated plates, but I harvest all floating and attached cells into the FACS tube and then proceed with the fixation/staining. Sorry for the basic question.

In principle, you need to count the cells and add the same amount of PI per cell to each sample so that all cells will have the same uptake of PI.