In Xenopus, a cytoplasmic agent known as MPF induces entry into mitosis. In fission yeast, genetic studies have shown that the cdc2 kinase regulates mitotic initiation. The 13 kd product of the suc1 gene interacts with the cdc2 kinase in yeast cells. We show that the yeast suc1 gene product (p13) is a potent inhibitor of MPF in cell-free extracts from Xenopus eggs. p13 appears to exert its antagonistic effect by binding directly to MPF. MPF activity is quantitatively depleted by chromatography on a p13 affinity column. Concomitantly, the Xenopus counterpart of the yeast cdc2 protein is adsorbed to the column. A 42 kd protein also binds specifically to the p13 affinity matrix. These findings suggest that the Xenopus cdc2 protein and the 42 kd protein are components of MPF.