The effect of the human cytomegalovirus immediate early region 1 enhancer on transcription was studied in vitro with HeLa cell nuclear extract. Stimulation of in vitro transcription mediated by the enhancer element involves its recognition by specific trans-acting factors present in the nuclear extract. DNase I protection analysis was used to determine at the nucleotide level those enhancer sequences that interact with nuclear factors. At least nine sites of protein-DNA interaction were detected over approximately 400 base pairs of enhancer sequence. The regions of nuclease protection are associated with 21-, 19-, 18-, and 17-base-pair repeat elements as well as with a unique sequence, creating a large nucleoprotein complex. The relationship between the protein binding and the activity of the immediate early region 1 enhancer is discussed.