Lytic infection with herpes simplex virus (HSV) results in the repression of most host cell protein synthesis but produces an increased activity of the cellular AP-1 transcription factor. This increase Is paralleled by an increase In the transcription rate of the proto-oncogene encoding the AP-1 component, c-Jun resulting in an increase in c-Jun protein In infected cells. The increased AP-1 activity in infected cells is dependent upon the HSV immediate-early protein ICPO. Thus a mutant lacking the gene encoding this protein falls to increase AP-1 activity whilst an ICPO expression plasmid can specifically increase the activity of an AP-1 dependent promoter in co-transfection experiments. The Implications of these effects in the interaction of HSV with cultured cells are discussed.