Streptavidin Magnetic Beads are 1 μm superparamagnetic particles covalently coupled to a highly pure form of streptavidin. The beads can be used to capture biotin labeled substrates including antigens, antibodies and nucleic acids (1,2). The strength of the biotin/streptavidin interaction, an association constant (Ka) of 1015 M-1 coupled with the low nonspecific binding of streptavidin permits captured substrates to be useful as ligands in subsequent experiments including mRNA isolation and the capture of primary or secondary antibodies.