The in vitro culture of animal cells commonly requires a medium rich in nutrients. These nutrients are usually supplied by supplementing a chemically defined medium with animal serum. 

Serum offers a highlybioavailable complex of proteins and amino acids, lipids and triglycerides, vitamins, inorganic minerals and salts, as well as other minor components and growth factors that promote culture success. Serum may also contain varying concentrations of ironsequestering heme and mitogenic endotoxin.