Endotoxin is a complex lipopolysaccharide (LPS) found in gram-negative bacteria's outer cell membrane and is typically waterborne. Bacteria shed endotoxin in large amounts upon cell death and when they are actively growing and dividing. Endotoxin is measured in Endotoxin Units per milliliter (E.U./mL). One EU/mL equals approximately 0.1 to 0.2 ng/mL. Endotoxin is directly related to the quality of collection and processing of serum; the more endotoxin, the more exposure to gram-negative bacteria.
The industry standard for endotoxin in fetal bovine serum is less than 10 E.U./mL; even at that level, it is suitable for most cell culture applications. Some exceptions are the production of vaccines and injectables, in which case the lowest levels are desirable. Some manufacturers will tell you that the lowest levels are better across the board and charge a premium for products that have levels typically below 1 E.U./mL. If your application does not require these low levels, you can probably save some money using standard grade material. There is a wealth of online information available regarding this subject.