Activation of the cdc2 protein kinase at different stages of the cell cycle is regulated by post-translational modifications and interactions with cyclins. We show that in vitro translated human cdc2 binds very poorly to A and B cyclins, unless it has been preincubated with a Xenopus egg extract. This results in the phosphorylation of cdc2 which allows binding to cyclins. The replacement of Thr161, a residue conserved and phosphorylated in other protein kinases, with valine inhibits cdc2 association with A and B cyclins. In addition, mutations in the amino-terminus of cdc2 and within the conserved 'PSTAIR' region strongly inhibit binding. The Thr161Val mutation causes a lethal phenotype in the fission yeast Schizosaccharomyces pombe, while replacement of Thr161 with glutamic acid, potentially mimicking phosphorylation, causes uncoordination of mitosis and multiple cytokinesis. These results suggest that a threonine phosphorylation/dephosphorylation cycle is involved in regulating cdc2 function.