Streptavidin SPA beads are designed for use in assay systems employing a biotinylated acceptor molecule or substrate and can be used in either capture or cleavage assay formats.

The binding of radiolabelled biotinylated compounds to the streptavidin SPA beads brings the isotope into close proximity to the scintillant. This allows the emitted radiation (beta-particles for [³H] or Auger electrons for [¹²⁵I]) to stimulate the scintillant to emit light. Any unbound radiolabelled ligand is not in close enough proximity to the scintillant to allow such energy transfer and hence no signal is generated. Light emitted by stimulated SPA beads can be detected by either conventional scintillation counters or multidetector instruments.