Methods are disclosed for determining an analyte in a medium suspected of containing the analyte. One method comprises providing (1) a medium suspected of containing the analyte, (2) a label reagent comprising a first specific binding pair (sbp) member associated with a photochemically activatable chemiluminescent compound wherein the first sbp member is capable of binding to the analyte or to a second sbp member to form a complex related to the presence of the analyte. The method further comprises photochemically activating the chemiluminescent compound. The amount of luminescence generated by the chemiluminescent compound is detected. The amount thereof is related to the amount of analyte in the medium. Compositions and kits are also disclosed.