As a prerequisite for the activation of MPF, the cdc2 protein kinase must undergo tyrosine dephosphorylation. Genetic studies have demonstrated that the cdc25 protein activates the cdc2 protein kinase once DNA replication has been completed. We have produced the cdc25 protein in bacteria and shown that it activates MPF in Xenopus extracts. In extracts that normally cannot enter mitosis owing to inhibition of DNA synthesis, the addition of active cdc25 protein efficiently elicits the mitotic state by inducing premature dephosphorylation of tyrosine on the cdc2 protein. The cdc25-dependent activation reaction can be reconstituted in a partially purified system lacking ATP. These biochemical experiments demonstrate that the cdc25 protein actively drives tyrosine dephosphorylation of the cdc2 protein and offer the prospect for characterizing the individual factors that regulate the activation of MPF during the progression from S phase to mitosis.