Biotin-PEG12-NHS ester is PEG derivative containing a biotin group and an NHS ester group. The hydrophilic PEG spacer increases solubility in aqueous media. The NHS group reacts specifically and efficiently with lysine and N-terminal amino groups at pH 7-9 to form stable amide bonds. The long PEG spacer arm also gives this reagent a long and flexible connection to minimize steric hindrance for binding to avidin molecules.