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Hi, This is a great question since sometimes for experimental reasons you might want to do this. Cells that are grown in a serum containing media can be transferred to a serum-free medium, D-PBS (or a serum-containing medium like D-MEM without the serum added) for about 4 hours without hurting them. Remember, if you want to keep your cells attached to the plastic you need to have calcium and magnesium in the medium at all times. Some reasons you might want remove serum is to control experimental conditions or to get the best response using some type of inducer, say for gene expression studies. This works by minimizing binding of an inducer compound to albumins contained in serum., meaning more inducer gets to the correct site of action. Note, that if you have cells conditioned to grow in a serum-free medium you should not add serum since this results in the addition of animal origin components that may be harmful downstream in another process.