A nuclear protein with affinity for the 5' flanking region of a cell cycle dependent human H4 histone gene has been partially purified from nuclear extracts of human HeLa S3 cells. The region involved in the binding of the protein has been localized to an upstream DNA segment using an electrophoretic mobility shift assay. This DNA segment is devoid of RNA polymerase II consensus sequences and contains both homopurinic and A/T rich tracts. Analogous experiments have identified a similar, and perhaps identical, factor that has affinity for a cell cycle dependent human H3 histone gene promoter. This protein appears to bind to a DNA segment containing A/T rich sequences that bear homology with the binding region of the H4 histone promoter. Cell synchronization experiments have shown that the overall affinity of the protein(s) for the H3 and H4 histone 5' flanking regions in vitro is not dramatically altered during the cell cycle. Although the rate of histone gene transcription is modulated during early S phase, transcription occurs throughout the cell cycle. Hence, the protein(s) we have detected here may play a role in the basal expression of these genes.