The cdc2 kinase and B-type cyclins are known to be components of maturation- or M-phase-promoting factor (MPF). Phosphorylation of cyclin B has been reported previously and may regulate entry into and exit from mitosis and meiosis. To investigate the role of cyclin B phosphorylation, we replaced putative cdc2 kinase phosphorylation sites in Xenopus cyclins B1 and B2 by using oligonucleotide site-directed mutagenesis. We found that Ser-90 of cyclin B2 and Ser-94 or Ser-96 of cyclin B1 are the main phosphorylation sites both in functional Xenopus egg extracts and after phosphorylation with purified MPF in vitro. Microtubule-associated protein (MAP) kinase from Xenopus eggs phosphorylated cyclin B1 significantly at Ser-94 or Ser-96, whereas it was largely inactive against cyclin B2. The substitutions that ablated phosphorylation at these sites, however, resulted in no functional differences between mutant and wild-type cyclin, as judged by the kinetics of M-phase degradation, induction of mitosis in egg extracts, or induction of oocyte maturation. These results indicate that the phosphorylation of Xenopus B-type cyclins by cdc2 kinase or MAP kinase is not required for the hallmark functions of cyclin.