The enhancer and promoter of the immediate early gene of human cytomegalovirus (hCMV) were tested as a transcriptional control element by fusion to the cat gene, followed by measurement of its expression from plasmid DNA in the extrachromosomal and integrated state, respectively. Comparison of the hCMV enhancer-promoter to two other viral elements was performed in six commonly used cell lines of different tissue and species origin. Irrespective of the cell line and of the state of the DNA, the hCMV enhancer-promoter was considerably stronger than both the SV40 promoter and the long terminal repeat of Rous sarcoma virus.