Two plant flavanones, (Sigmoidin A and B) having noticeable antibacterial activity, were assayed using a preparation for the study of sea urchin egg cleavage. When added after insemination, both toxins inhibit egg division with a half maximal dose of 7.5 microM for Sigmoidin A and 12 microM for Sigmoidin B. The first Ca2+ signal following fertilization was not modified by the molecules, however, the intracellular storage of calcium in isolated non-mitochondrial compartments was reduced in a dose-dependent manner by Sigmoidin A and Sigmoidin B. Both trigger a complete discharge of the sequestered calcium. In vivo the flavanones dramatically reduced the capacity of storage of non-mitochondrial intracellular calcium compartments necessary to the cyclical elevation of cytosolic free calcium during the cell cycle.