Maturation promoting factor (MPF) has been shown to induce meiosis or mitosis when injected into Xenopus oocytes and embryos. We show here that early events of mitosis, chromatin condensation and nuclear envelope breakdown, are induced by MPF in somatic interphase nuclei incubated in a cell-free extract of Xenopus eggs. These events occur rapidly and synchronously in response to MPF and are reversed when MPF activity is diminished. Using this cell-free system, we show that major structural proteins of the nuclear lamina, lamins A and C, are hyperphosphorylated within 15 min after addition of MPF, followed by a gradual depolymerization of the nuclear lamina until the nuclear envelope breaks down 30 min later. These results show that MPF induces mitotic events in vitro, and that phosphorylation of the lamins precedes disassembly of the nuclear lamina and could act to trigger nuclear envelope breakdown.