Germinal-vesicle-breakdown (GVBD) was induced if a 132,000-g supernatant of Tetrahymena thermophila homogenates was injected into Xenopus oocytes. Using this induction of GVBD as a bioassay system, a GVBD-inducing substance was purified from the Tetrahymena by ultra-filtration, liquid chromatography, and electroelution from a band on native-PAGE gel. Proteins eluted from the single band on the native-PAGE gel induced GVBD in the absence of oocyte protein synthesis. This band resolved into two bands on SDS-PAGE: 60 and 112 kDa. The 60 kDa protein was the active fraction inducing GVBD. Immunoprecipitation of the 60 kDa protein prevented the GVBD-inducing activity, supporting the conclusion that the 60 kDa protein is the GVBD-inducing substance. An immunoblot with anti-60 kDa monoclonal antibody and PSTAIR antibody showed that p13^suc1-beads could remove cdc2 homologues from T. thermophila supernatant but could not remove the GVBD-inducing activity. The 60-kDa protein appeared at the same time as micronuclear division and disappeared at the beginning of the macronuclear division during synchronous cell division. The cyclic appearance of the 60-kDa protein in the T. thermophila cell cycle suggests that this protein has a cell cycle function.