The immediate-early (IE) genes of herpes simplex virus type 1 (HSV-1) are the first to be expressed during infection in tissue culture. Since they are transcribed at abnormally high levels in the absence of IE protein synthesis they appear to be subject to repression during normal infection. One of the major HSV-1 regulatory proteins, Vmw175 (the product of IE gene 3), is required for normal IE gene regulation since mutations which inactivate it lead to abnormally high levels of IE gene expression. The mechanism of repression of the IE-3 promoter requires both the ability of Vmw175 to bind to DNA and the presence of a Vmw175 recognition DNA binding sequence at the cap site of the IE-3 promoter. A similar Vmw175 DNA binding sequence has been defined within the IE-1 promoter. This paper describes the construction of a variant of HSV-1 with a mutation within the IE-1 Vmw175 DNA binding site. Although the mutation destroyed the ability of Vmw175 to bind to the site, and greatly reduced the ability of Vmw175 to repress the IE-1 promoter in transfection assays, the mutation had no effect on the levels of Vmw110 expression during normal HSV-1 infection.