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Transitioning your cells from one media to another or from adherent to suspension growth things can get tricky. I assume you are transitioning in a serum containing media like Ham’s F12 for your CHO cells, since serum-free media is generally made for suspension culture. You want to use early passage cells if possible. Seed your cells at a higher density than normal. My suggestion is to use Ham’s F12 without calcium or magnesium and some people reduce the serum from 10% to 5%. Also you might want to add a low concentration of the detergent Pluronic F-68 to reduce shear forces. Use a sterile shaking flask without baffles and shake at about 125 rpm. Good luck. Keep culturing your cells and the viability should increase as selection occurs.