A protein complex responsible for triggering mitosis in somatic cells and for maturation of oocytes into egg cells. Consisting of cyclin B (see cyclin) bound to a cyclin-dependent kinase (CDK), it catalyses the phosphorylation of proteins that in turn bring about the events of mitosis, including condensation of chromosomes, formation of the mitotic spindle, and breakdown of the nuclear envelope. Levels of cyclins and MPF rise as the cell enters mitosis, reach a peak during mitosis, and then fall during anaphase.