The binding of a number of extracellular ligands (hormones, growth factors, neurotransmitters etc.) to their plasma membrane receptors causes hydrolysis of phosphatidylinositol bisphosphate to initiate the formation of two second messengers, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and diacylglycerol, DAG. DAG has been shown to activate protein kinase C, whereas Ins(1,4,5)P3 induces the release of Ca2+ from an intracellular pool. This rapidly mobilizable, Ins(1,4,5)P3-sensitive Ca2+ store has until now been identified as the endoplasmic reticulum, ER. We demonstrate that this is untenable and provide evidence for the existence of an unrecognized organelle, the 'calciosome'. This conclusion is based on the following experimental evidence. (1) There is no correlation between the abundance of ER and the amount Ins(1,4,5)P3-sensitive Ca2+ release. (2) There is no correlation between ER markers and those for the Ca2+ store [Ins(1,4,5)P3 binding and sensitivity, Ca2+ uptake]. (3) A protein similar to striated muscle calsequestrin, CS, has been identified in microsomal fractions from a number of tissues; it copurifies with markers of the Ca2+ store, but not with those of ER. (4) Subcellular localization of the CS-like protein by electron microscopy reveals that in all cells so far analysed this protein is localized in small, membrane-enclosed structures, calciosomes, which are also stained by an anti-Ca2+-ATPase antibody. Calciosomes appear to be morphologically distinct from any other known cell organelle. (5) Although they stain different portions of the calciosomes (membrane and lumen, respectively), anti-Ca2+-ATPase and anti-CS antibodies do not recognize any antigen in ER cysternae; antibodies directed against known components of ER do not bind to calciosomes.