Serum is the blood component that is obtained after coagulation and removal of cellular components. Besides serum proteins, it contains, e.g. growth factors, amino acids, and hormones. This complex mixture makes serum one of the most important supplements supporting the cells growth and proliferation for in vitro cell culture. Of special interest is the Fetal Bovine Serum (FBS). It is especially rich in growth factors and is particularly low in antibodies that may negatively influence the cell culture work. It supports most cell lines and undergoes strict quality controls. Some applications such as lipid-metabolism studies or Arteriosclerosis research need reduced lipid concentrations in FBS. To remove lipids from FBS, we use the fumed silica precipitation method. When fumed silica is added to serum, non-polar substances such as lipids and steroids adsorb to the surface of the silica. The solution is then centrifuged and the pellet of fumed silica with adsorbed lipids is removed. The serum is then sterile-filtered. The acceptance criterion for the treatment is a level of cholesterol lower than 10 mg/100 ml. Lipids and other non-polar substances are reduced in comparison to non-treated serum.