Herpes simplex virus type 1 encodes five immediate-early gene products, at least three of which are required for fully efficient viral gene expression. One of these three, Vmw110 (or ICPO), is a potent and nonspecific activator of gene expression in transfection assays. Viruses which fail to express functional Vmw110 have a cell-type and multiplicity-dependent defect in viral gene expression. In addition, Vmw110 has been implicated in the reactivation of latent virus in mouse and tissue culture model systems. In this report we show that Vmw110 can be isolated as a fast-sedimenting complex from infected cells and that this complex contains a cellular protein which is immune precipitated in association with Vmw110. This association can be reconstructed by adding purified Vmw110 to lysates of several different cell types. By using a GST fusion protein, we have found that the isolated C-terminal portion of Vmw110 can complex strongly and specifically with a similar cellular protein. The relevance of this observation to the roles of Vmw110 in latent and lytic virus infection is discussed.