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, which may negatively influence the cell culture work. It supports most cell lines and undergoes strict quality controls. Cell culture applications with tet-inducible gene expression systems and other applications where the presence of tetracycline would be disturbing require the use of a tetracycline-negative FBS. Capricorn Scientific has established a test with an external laboratory, using liquid chromatography-tandem mass spectroscopy to detect all kinds of tetracycline. This method allows accurate detection of Chlortetracycline and Oxytetracycline in a range of 0.05 mg/l.