The cells which are shown in the HL-60 optimized protocol are living cells and not fixed cells. However a good method for fixation of GFP is to use 3.5%-4% Paraformaldehyde in PBS. Please keep in mind that the fixation method is dependant on what you would like to stain. Do not use Formaldehyde as it is not suitable for fixation. Please note that HL-60 cells are not adherent cells. Therefore, we recommend using coated coverslips for fixation so you will not wash away your cells. In general, the coverslips should be coated prior to fixation with poly-L-lysine, for example.