Treatments which inhibit or inactivate Cdk1/cyclin B in metaphase-arrested mammalian cells and budding yeast are described. These treatments induce the cells to exit mitosis and return to interphase, though without chromosome segregation or cytokinesis, and they provide the basis for a method to identify enzymes or other proteins which act “downstream” from Cdk1 inactivation and to elucidate the roles of those proteins in mitotic exit. In this method, inactivation of Cdk1 is combined with inhibition or inactivation of a protein of interest and the effects are observed. This approach should be particularly useful for determining which protein phosphatases are involved in the transition from mitosis to G1-phase and for identifying their substrates.