Although considerable evidence has accumulated linking herpes simplex virus (HSV) to carcinoma of the uterine cervix, only fragmentary evidence exists linking HSV to oral cancer. The purpose of this study was to search for HSV type 1 (HSV-1) proteins in tissue sections of 132 oral squamous cell carcinomas and 56 age- and site-matched controls. All tissues were incubated in rabbit anti-HSV-1 serum with the use of a modified direct immunoperoxidase technique. More than 42% of the cancers, but none of the controls, reacted positively. Thirteen selected oral carcinomas that exhibited positive polyclonal staining were incubated with the use of a peroxidase-antiperoxidase technique with a mouse monoclonal antibody raised against the 89 kilodalton HSV-1 envelope protein. None of these tissues exhibited a positive reaction. Similarly, no unequivocal ultrastructural viral particles were found in 11 oral carcinomas that stained positively with the polyclonal antibodies. These results are interpreted to indicate partial expression of a noninfectious viral genome. The presence of this genome in a high proportion of lesions in this study lends strong statistical support to the oncogenic relationship between HSV-1 and oral squamous cell carcinoma.