cdc2 mRNA transcripts were detected in immature bone marrow cells and became undetectable along with differentiation. Peripheral blood resting cells did not express cdc2 mRNA, but it was induced in T-lymphocytes when the cells reentered the cell cycle in response to specific mitogens. In contrast, cdc2 mRNA could not be induced in granulocytes and monocytes even after the culture with the appropriate stimulants. In order to investigate the mechanism of the regulation of cdc2 mRNA expression in hematopoietic cells, we isolated the 5'-flanking sequence of the cdc2 gene and found the putative E2F binding site at the position of nucleotides -124 to -117. The binding of E2F at this region was detected by a gel-retardation assay and DNaseI footprinting in phytohemagglutinin-stimulated T-lymphocytes, which was coincident with the expression of cdc2 mRNA. E2F binding was not observed in both granulocytes and monocytes. Transient chloramphenicol acetyltransferase assay revealed that the region containing E2F binding site had a strong promoter activity, and introduction of the mutation at the E2F binding site resulted in a significant loss of the activity. E2F-1 and DP-1 mRNAs were not detectable in granulocytes, monocytes and resting T-lymphocytes but were induced after the mitogenic stimulation of T-lymphocytes. The induction of E2F activity preceded the appearance of cdc2 mRNA, which is consistent with the role of E2F in the regulation of cdc2 mRNA expression. These results suggest that cdc2 mRNA expression is related to the cell cycling of normal human hematopoietic cells and that E2F plays some roles in the regulation of its expression.