Microinjection of a crude yeast extract into fully grown immature Xenopus laevis oocytes induces the reinitiation of meiotic maturation as does the X. laevis oocyte MPF (maturation promoting factor) by itself. Using several strains of yeast exhibiting temperature-sensitive cell cycle block (cdc), yeast extract exhibited MPF activity only when obtained during the time interval from the G2 period to the metaphase. The eukaryotic ubiquity of MPF is stressed.