Herpes simplex virus encodes a 175-kilodalton immediate-early transactivating protein referred to as ICP4. A mutant ICP4 molecule expressed from a stable transformed cell line lacks the sequences required for transactivation yet retains the ability to specifically associate with DNA and to form homodimers. Expression of the mutant ICP4 peptide from this cell line, designated X25, resulted in the inhibition of herpes simplex virus growth. Wild-type ICP4 homodimers were depleted in X25-infected cells by the formation of heterodimers containing the wild-type ICP4 molecule and the mutant peptide. While the ICP4 heterodimer retained DNA-binding activity, immunological studies suggest that the wild-type subunit of the heterodimer is conformationally altered in a region that serves as the antigenic epitope. Physical studies that determined the composition of the heterodimer and its native size and approximate shape support this observation. The structural change is in a region of ICP4 genetically implicated as important for transactivation and may result in an alteration in an interaction between ICP4 and a target protein essential to promote transcriptional activation. Sequestering wild-type monomers of a viral regulatory protein into heterodimers which are less proficient in transactivation may explain the dominant inhibitory activity of the X25 cells, resulting in attenuation of viral growth.