The maturation-promoting factor (MPF) is a regulatory factor that initiates mitosis by phosphorylating a wide variety of structural and regulatory proteins. MPF consists of a regulatory component, cyclin B, and a catalytic component, the protein kinase p34cdc2 and its activation requires phosphorylation of p34cdc2 on Thr161, and dephosphorylation on Tyr15 and possibly also Thr14. In Xenopus oocytes there is a stock of inactive cyclin B/p34cdc2 complexes (pre-MPF) in which p34cdc2 is phosphorylated on Thr161, Tyr15 and Thr14. The activation of pre-MPF requires dephosphorylation of Tyr15 and probably also Thr14, and these dephosphorylations are thought to be instrumental in the activation of MPF during oocyte maturation . The direct upstream regulators of MPF are the kinase weel and the phosphatase cdc25. The aim of this project is to identify and characterise the protein kinases and phosphatases involved in the activation of MPF during Xenopus oocyte maturation, studying kinases and phosphatases implicated in the regulation of weel and cdc25, as well as the role(s) of cyclic AMP-dependent protein kinase (PKA) in the maturation. The activities that regulate cdc25 and weel in Xenopus oocytes are likely to be widely conserved in other species, and may also have a more general role in the regulation of mitosis in somatic cells. Experimentally, Xenopus oocytes and eggs are an excellent system for the biochemical characterisation and purification of kinases and phosphatases.