It has been known in amphibians and starfishes that a cytoplasmic factor called maturation-promoting factor (MPF), produced in maturing oocytes under the influence of the maturation-inducing hormones, can induce germinal vesicle breakdown (GVBD) and the subsequent process of meiotic maturation. The present study revealed that injection of cytoplasm of maturing starfish oocytes (starfish MPF) into immature sea cucumber oocytes brought about maturation of the recipients. Amphibian MPF obtained from mature oocytes of Xenopus laevis or Bufo bufo was found to induce maturation of starfish oocytes following injection. Cytoplasm taken from cleaving starfish blastomeres induced maturation when injected into immature starfish oocytes. The maturation-inducing activity of cytoplasm of starfish blastomeres changed along with the mitotic cell cycle during 1- to 4-cell stages so far tested and reached a peak just before cleaving. Furthermore, an extract of mammalian cultured cells, CHO or V-79, synchronized in M phase, induced GVBD in starfish oocytes following injection, whereas S phase extract had little activity. These facts suggest that MPF generally brings about nuclear membrane breakdown in both meiosis and mitosis, and that the nature of MPF is very similar among vertebrates and invertebrates.