The herpes simplex virus trans-activator Vmw65 interacts with the cellular factors Oct-1 and VCAF-1 to generate a multicomponent DNA binding complex that specifically recognizes conserved enhancer elements found upstream of the viral immediate-early genes, resulting in potent stimulation of their transcription. We have identified a HeLa cell factor, distinct from Oct-1 or VCAF-1, which significantly enhances the stability or formation of Vmw65-dependent complexes, as judged by mobility shift analysis using either nuclear extracts or bacterially expressed Oct-1 and Vmw65. This factor, designated SF (stimulatory factor), was partially purified from HeLa cell postnuclear extracts and has an apparent molecular weight of 1500-3000, based on ultrafiltration and size-exclusion chromatography. SF was shown to be resistant to inactivation by heat treatment, protease, nuclease, and phospholipase digestions, and extraction with organic solvents. Pretreatment of SF with beta-glucuronidase did not affect its ability to stimulate Vmw65-dependent complex formation but did reduce the electrophoretic mobility of the resulting complex. These data indicate that SF is probably a component of the Vmw65-induced complex and may be composed, at least partially, of carbohydrate.