Our streptavidin is isolated from Streptomyces avidinii by a specific affinity system which results in a homogeneous, ultrapure streptavidin with very low nonspecific binding characteristics. Unconjugated streptavidin is recommended for use in solid-phase assays and immunohistochemical applications when the technique of sequential addition of reagents (“sandwich technique”) is employed. This reagent has also been effectively employed in subtractive hybridization procedures.