We have examined the sequences required in vivo to promote transcription of a cell cycle-regulated human H4 histone gene. Deletion mutants of the 5' flanking region were assayed in mouse cells or fused with the chloramphenicol acetyltransferase (CAT) gene for assay in HeLa cells. The functional limits of the regulatory sequences were shown to extend at least 6.5 kilobases (kb) upstream. Sequences sufficient for correctly initiated transcription were found in the 70 base pairs (bp) immediately 5' to the cap site. A proximal element located 200-400 bp upstream increased the level of transcription several times above the basal level, although not to maximal levels. Maximal levels of expression were achieved with 6.5 kb of 5' flanking sequence adjacent to the proximal promoter sequences or when a distal enhancer element with both position- and orientation-independent function was moved proximal to the promoter. Our results indicate that a series of 5' cis-acting sequences are functionally related to the fidelity and level of expression of this human H4 histone gene.