Albumin is mostly a simple hydrophilic protein present in cells and body fluids. Albumin is synthesized in the liver, and serum albumin (69 kDa, pI 4.9) occupies 56–60% of total serum proteins. Because of its large population, albumin is very important in maintaining plasma osmotic pressure. Albumin can bind hydrophobic physiological substances e.g. fatty acids, bilirubin, and thyroxine and contributes the transfer of these substances. Bovine albumin is generally thought to be an inert protein and is commonly used in various incubation media, and especially calf serum is very important component of tissue culture media. This means that if some substances are prepared using tissue- or cell-culture technique, they possible contaminated with bovine albumin. In order to assure product purity bovine albumin in the product has to be estimated. The present highly sensitive ELISA kit will be useful in this kind of estimation.