WGA-PEI Type A or B SPA Imaging beads are designed to be used in receptor ligand binding assays. During assay incubation carbohydrate residues present in cell membranes bind to WGA on the SPA Imaging bead, effectively immobilizing the receptor-bearing membranes on to the SPA Imaging bead.

The binding of radiolabelled ligands to such immobilized receptors brings the isotope into close proximity with the scintillant, which is incorporated within the bead. This allows the emitted radiation (betaparticles 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 Imaging beads can be detected by imaging systems, with suitable filters.