Transfection of plasmids containing the herpes simplex virus type 1 (HSV-1) alpha gene 27 has been observed to inhibit gene expression from a virus alpha promoter in monkey (CV-1) but not human (HeLa) cells in a transient gene expression system. DNA mediated gene transfer to CV-1 and HeLa cells of the bacterial gene for chloramphenicol acetyl transferase (CAT) linked to the promoter regulatory domain from the HSV-1 alpha 4 gene results in production of substantial levels of CAT enzyme. Cotransfection of equal mole amounts of an alpha 27 containing plasmid with an alpha 4-CAT construct to CV-1 cells results in a greater than 85% inhibition of CAT activity. No significant inhibition of CAT activity was observed when transfection was done in HeLa cells, with the same concentrations tested. Intact alpha 27 structural genes were necessary to achieve inhibition since subgenomic fragments and restriction enzyme digested alpha 27 genes were not effective inhibitors. Cotransfection of alpha 27 genes to CV-1 cells also prevented alpha 0 as well as alpha 4 from mediating their trans-stimulation of the HSV thymidine kinase (tk) regulated CAT gene, B-CAT. This suggests that the alpha 27 gene product may down-regulate gene expression from alpha promoters.