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. While the whole serum is acceptable for routine processes, studies involving nutritional parameters or incorporation of labeled material (e.g. radioactive amino acids) require the precise removal of certain constituents. The most commonly used method for removal of these constituents is the dialysis of whole serum. Dialysis is performed with a cut-off of 10 kDa.