In the fission yeast S. pombe, the Mr = 34 kd product of the cdc2+ gene (p34cdc2) is a protein kinase that controls entry into mitosis. In Xenopus oocytes and other cells, maturation-promoting factor (MPF) appears in late G2 phase and is able to cause entry into mitosis. Purified MPF consists of two major proteins of Mr approximately equal to 32 kd and 45 kd and expresses protein kinase activity. We report here that antibodies to S. pombe p34cdc2 are able to immunoblot and immunoprecipitate the approximately equal to 32 kd component of MPF from Xenopus eggs. The Mr approximately equal to 32 kd and 45 kd proteins exist as a complex that expresses protein kinase activity. These findings indicate that a Xenopus p34cdc2 homolog is present in purified MPF and suggest that p34cdc2 is a component of the control mechanism initiating mitosis generally in eukaryotic cells.