Methods and reagents are disclosed for conducting assays. Embodiments of the present methods and reagents are concerned with chemiluminescent reagents for determining the presence and/or amount of an analyte in a sample suspected of containing the analyte. The reagent is non-particulate and comprises a binding partner for the analyte and a chemiluminescent composition comprising an olefinic compound and a metal chelate. In embodiments of an assay, a combination is provided that comprises a sample suspected of containing the analyte, a chemiluminescent reagent as described above and a sensitizer reagent capable of generating singlet oxygen. The combination is subjected to conditions for binding of the analyte to the binding partner for the analyte. The sensitizer is activated and the amount of luminescence generated by the chemiluminescent composition is detected wherein the amount of luminescence is related to the amount of the analyte in the sample.