Methods and reagents are disclosed for conducting assays. Embodiments of the present methods and reagents are concerned with a solid support such as, for example, a particle. The support includes a chemiluminescent composition that includes a metal chelate. The present inventors observed that, when such support such as, e.g., particles, were employed in assays for the determination of an analyte, stability of signal output by the chemiluminescent composition associated with the particle was unacceptably reduced as compared to particles including other chemiluminescent compositions. In accordance with embodiments of the present invention, the stability of signal output from such particles is enhanced by including in a medium that contains the particles a sufficient amount of one or more stabilizing agents, which may be a chelating agent and/or a metal chelate such as, for example, the metal chelate that is associated with the particle.