Antigens induced by herpes simplex virus Type 2 (HSV2) were found to be associated with squamous-cell carcinoma in situ of the vulva in nine of 10 patients. The HSV2-induced antigens are DNA-binding proteins that are normally present in the nuclei of infected cells, but in the cells of the carcinomas in situ they were found in the cytoplasm. Whole-virion structural antigens were not present, although there was serologic evidence of previous HSV2 infection in patients tested for the presence of antibodies. The observations reported here and the recent parallel rise in the prevalence of both HSV2 infections and vulvar carcinoma in situ, particularly in women under 40 years of age, suggest an association of HSV2 infection with this type of neoplasia, the nature of which remains to be determined.