A composition for use in a thermochemiluminescent assay. The composition includes a heme protein, a luminescer such as luminol, and serum albumin, the luminescer being non-covalently bonded to serum albumin. The composition may be used in a thermochemiluminescent for the ligant of a specific binding pair in which the composition is contacted with a fluid sample suspected of containing the antiligand of the specific binding pair and the subsequent light-emitting reaction is quantified. The specific binding light-emitting reaction is quantified. The specific binding pair can be, for instance, an enzyme and the substrate for which it is specific. The composition and the assay can be modified to detect ionizing radiation, peroxidative injury to cells, heat or a particular cell type.