Cytoplasmic extracts of HeLa cells synchronized in various phases of the cell cycle were injected into fully grown Xenopus laevis oocytes to monitor the presence of factors that can induce meiotic maturation: i.e., germinal vesicle breakdown and chromosome condensation. Extracts from GI and S phase cells had no activity. The maturation-inducing activity, which was found to be low during early and mid G2 phases, increased rapidly during late G2 and reached a peak in mitosis. The results of this study suggest that the factors that regulate the breakdown of nuclear membrane and chromosome condensation during mitosis, meiosis, and premature chromosome condensation appear to be very similar, if not identical, throughout the animal kingdom.