Antibiotic that acts as a potent and selective calcium ionophore; more effective than A23187.  Ionomycin binds Ca2+ in the 7.0-9.5 pH range with the resulting complex exhibiting intense UV absorption. Ionomycin induces apoptosis in immature B cell lines and in cultured embryonic rat cortical neurons.  More mature B cell lines do not undergo apoptosis, but are arrested in the G1 phase of the cell cycle.