Water-soluble discrete multi-biotin-containing compounds with at least three (3) biotin moieties are disclosed. The water-soluble biotin-containing compounds may additionally comprise one or more moieties that confer resistance to cleavage by biotinidase or that is cleavable in vitro or in vivo. The discrete multi-biotin-containing compounds may include a reactive moiety that provides a site for reaction with yet another moiety, such as a targeting, diagnostic or therapeutic functional moiety. Biotinylation reagents comprising water-soluble linker moieties are also disclosed and may additionally comprise a biotinidase protective group. Methods for amplifying the number of sites for binding biotin-binding proteins at a selected target using multi-biotin compounds also are disclosed.