At fertilization in mammals the sperm triggers a series of oscillations in intracellular Ca 2+ within the egg. These Ca 2+ oscillations activate the development of the egg into an embryo. It is not known how the sperm triggers these Ca 2+ oscillations. There are currently three different theories for Ca 2+ signaling in eggs at fertilization. One idea is that the sperm acts as a conduit for Ca 2+ entry into the egg after membrane fusion. Another idea is that the sperm acts upon plasma membrane receptors to stimulate a phospholipase C (PLC) within the egg which generates inositol 1,4,5-trisphosphate (InsP 3 ). We present a third idea that the sperm causes Ca 2+ release by introducing a soluble protein factor into the egg after gamete membrane fusion. In mammals this sperm factor is also referred to as an oscillogen because, after microinjection, the factor causes sustained Ca 2+ oscillations in eggs. Our recent data in sea urchin egg homogenates and intact eggs suggests that this sperm factor has phospholipase C activity that leads to the generation of InsP 3 . We then present a new version of the soluble sperm factor theory of signaling at fertilization.