The crucial role of herpes simplex virus type 1 immediate early protein Vmw175 (ICP4) in the regulation of all classes of viral genes has been established by extensive analysis of temperature-sensitive, insertion and deletion mutants. It has long been known that Vmw175 binds to selected DNA sequences, and recent studies have shown that it interacts with components of the basal transcription machinery. However, the role of DNA binding in its mechanism of action has been controversial. Despite the presence of Vmw175 recognition sites at numerous locations throughout the viral genome, it has proved difficult to establish that these sites are important for the activation of early and late promoters. In this study we have analysed the ability of a large number of insertion and single point mutant derivatives of Vmw175 to bind to DNA and to support virus replication. Vmw175 mutants which were unable to bind to DNA were also incapacitated in terms of their ability to activate gene expression in transfection assays and to complement the growth of a Vmw175 deletion mutant virus. These results strongly support the hypothesis that interaction with DNA is an essential feature of the mechanism of action of Vmw175.