Injection of sperm extracts triggers Ca 2 + oscillations in mammalian eggs similar to those seen at fertilisation. Here, we show that addition of sperm extracts to sea urchin egg homogenates causes Ca 2 + release and inositol 1,4,5-trisphosphate (InsP 3 ) production. Furthermore depleting homogenates of phosphatidylinositol lipids using a phosphatidylinositol-specific phospholipase C blocked the sperm extract from causing InsP 3 production and a Ca 2 + rise. A response could be recovered by the addition of phosphatidylinositol 4,5-bisphosphate to either sperm extracts or egg homogenates. These data indicate that sperm extracts contain an InsP 3 -generating phospholipase C which may play a role in Ca 2 + release at fertilisation.