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The first step is to identify a serum-free media formulation compatible with the cell line you are using. Typically serum is removed from cultures in a gradual, step-wise approach, replacing the serum containing media with the new serum free media with reduced levels of FBS in it. Wean the cells from the FBS by replacing with serum free media using FBS at about 75% of the original concentration and allow the cells to recover. Repeat with media containing 50% and 25% of the original FBS concentration until you are in completely serum-free conditions. If at any step cell health starts to be negatively impacted, go back to the previous concentration of serum and allow the cells to recover for a longer period of time before further reducing the FBS.