The finding of a nuclear antigen by anticomplement immunofluorescence in cells treated with cytosine-arabinoside after infection of Herpes Simplex Virus (HSV), opens a new approach to the problem of the role of this virus in certain human cancers. Complement-fixing tests of HSV markers with cancer and control human sera as well as with hyperimmune guinea pig antisera are discussed, suggesting another parameter for studies of squamous cell carcinomas. The finding of HSV antigens in selected tumors as the expression of repressed viral genome proves a continuing release of virus specific message and supports the important role of the virus in the development of the tumor.