The chicken lysozyme gene was inserted into an SV40-based plasmid vector, and the recombinants were transfected into the human cell lines HeLa and MCF-7. Correct and efficient transient expression directed by the lysozyme promoter was found in both of these cell lines, as determined by S1 nuclease mapping and Northern blot analysis of the RNAs made. SV40 sequences dramatically enhance the expression of the lysozyme gene. This enhancing effect is only acting in cis and is distance/orientation dependent, since clones containing the lysozyme gene in either orientation produce different amounts of correct lysozyme transcripts. The transfected lysozyme gene was not induced by steroid hormone treatment of the cells.