Abstract: The present invention relates generally to glutathione derivatized beads which are adapted for use in conjunction with glutathione-S-transferase fusion proteins (generally, GST fusion proteins, which contain a fluorescent label such as fluorescent green protein) for use in flow cytometry. The present invention also relates to methods for detecting and/or quantifying interactions between a GST fusion protein and their binding partners, in particular, labeled binding partners such as fluorescently labeled binding partners. By creating glutathione beads with an appropriate high or increased site density, disadvantages often associated with low affinity systems and quick off-rates in solution may be resolved to provide a workable system and method. Methods of identifying potential agonists, antagonists and regulator compounds of proteins fused to GST from libraries of compounds represents another aspect of the present invention.