Biotin, vitamin and co-factor in a number of key reactions, binds very tightly to the tetrameric protein avidin (or streptavidin), with a dissociation constant Kd in the order of 10−15, which is one of the strongest biological non-covalent interactions. The biotin-avidin interaction is widely used in biochemistry, immunology and immunohistochemistry. Endogenous biotin present in biologicals samples, cells or tissues can be the source of false-positive signal when using the streptavidin biotin detection system. The AnaPrepTM Biotin Blocking Kit is optimized for blocking endogenous biotin in the cells or tissues. In the first step, addition of streptavidin blocks all endogenous biotin moieties. The remaining streptavidin is then blocked with free biotin. This results in the binding of streptavidin to all endogenous biotin molecules, and biotin to the added streptavidin.