In mammalian eggs, the fertilizing sperm evokes intracellular Ca 2 + ([Ca 2 + ] i ) oscillations that are essential for initiation of egg activation and embryonic development. Although the exact mechanism leading to initiation of [Ca 2 + ] i oscillations still remains unclear, accumulating studies suggest that a presently unknown substance, termed sperm factor (SF), is delivered from the fertilizing sperm into the ooplasm and triggers [Ca 2 + ] i oscillations. Based on findings showing that production of inositol 1,4,5-trisphosphate (IP 3 ) underlies the generation of [Ca 2 + ] i oscillations, it has been suggested that SF functions either as a phospholipase C (PLC), an enzyme that catalyzes the generation of IP 3 , or as an activator of a PLC(s) pre-existing in the egg. This review discusses the role of SF as the molecule responsible for the production of IP 3 and the initiator of [Ca 2 + ] i oscillations in mammalian fertilization, with particular emphasis on the possible involvement of egg- and sperm-derived PLCs, including PLCζ, a novel sperm specific PLC.