The photoprotein aequorin has a long history of being used as a sensitive intracellular probes for Ca2+. When three Ca2+ ions bind to it, aequorin emits blue light and decomposes itself into apoaequorin, coelenteramide and CO2. Inversely, apoaequorin can regenerate into the aequorin by incubation with coelenterazine in the presence of O2. The approximately third-power dependence of aequorin’s bioluminescence on Ca2+ concentration allows the measurement of Ca2+ concentrations with a broad detection range from 0.1 mM to >100 mM. Unlike fluorescent Ca2+ indicators, Ca2+-bound aequorin can be detected without illuminating the sample, thereby eliminating interference from autofluorescence. This Coelenterazine Sampler Kit consists of coelenterazine and four of its derivatives, 25 ug each, for reconstituting aequorin in cells that have been transfected with apoaequorin DNA. The coelenterazine analogs confer different Ca2+ affinities and spectral properties on he aequorin complex.