The transcription factor AML1 (RUNX1) plays a critical role in normal hematopoiesis and in the development of leukemia. The phenotype generated by knockout of the AML1 gene demonstrates that AML1 is necessary for normal development of fetal hematopoietic cells. Conditional knockout of AML1 expression in adult bone marrow leads to lineage-specific effects on B- and T-cell maturation and pronounced inhibition of common lymphocyte progenitor production. In addition, the AML1-deficient adult mice exhibited inefficient platelet production and a mild myeloproliferative phenotype characterized by an increase in peripheral blood neutrophils, an increase in myeloid progenitor populations, and extramedullary hematopoiesis composed of maturing myeloid and erythroid elements. Previous studies have shown that phosphorylation of AML1, particularly at serines 276 and 303, can affect transcriptional activation. We have now shown that phosphorylation of AML1 serines 276 and 303 can be blocked in vivo by inhibitors of the cyclin-dependent kinases (Cdks) Cdk1 and Cdk2. In addition, these two sites can be phosphorylated in vitro by purified, active Cdk1/cyclin B and Cdk2/cyclin A, but not by Cdk4/cyclin D. Mutation of AML1 serines 276 and 303 reduces AML1 activity during most phases of the cell cycle. In addition, mutation of serines 276 and 303 prevents phosphorylation from occurring at two nearby sites (serine 293 and threonine 300). Mutation of serine 293 and threonine 300 causes an increase of AML1 activity, especially in cells arrested in G2/M by treatment with nocodazole. Since Cdk phosphorylation can be stimulatory (serines 276 and 303) or inhibitory (serine 293, threonine 300), the interplay of Cdk phosphorylation at different sites might confer a very fine level of regulation of AML1 activity.