The results of complement fixation tests on 202 sera from people without cancer and from patients with cancer in 29 different areas of the body indicated that only those with nine varieties of advanced cancer (lip, mouth, oropharynx, nasopharynx, kidney, urinary bladder, prostate, cervix uteri, and vulva-all of 56 tested) gave positive specific reactions with nonvirion antigens induced by the DNA herpes simplex (HSV 1) and herpes genitalis (HSV 2) viruses. None of 57 people without cancer (including 10 with current and 18 with recurrent HSV 1 or HSV 2 infections), none of 81 patients with 20 other varieties of advanced cancer (gum, tongue, tonsil, salivary gland, accessory sinus, epiglottis, lung-bronchus, stomach, colon, breast, corpus uteri, ovary, testis, liver, thyroid, Wilms' embryonal kidney, melanoma, Hodgkin's disease, acute lymphocytic leukemia, and acute myelocytic leukemia), and none of four women with early malignant changes in the cervix uteri gave positive results. The seven patients with advanced cancer of the lip or oropharynx gave positive reactions with HSV 1 but not with HSV 2 nonvirion antigens (compatible with involvement of only HSV 1), all of the 13 women with advanced cancer of the cervix uteri and the one woman with advanced cancer of the vulva gave positive reactions with both HSV 1 and HSV 2 nonvirion antigens (compatible with involvement of only HSV 2), while among the 35 other positive patients only two (one with cancer of the kidney and one with cancer of the bladder) reacted with HSV 1 and not at all with HSV 2 nonvirion antigens. Positive sera failed to react with cells harvested at different times after high-multiplicity infection with the DNA vaccinia virus. Massive absorption of positive sera with trypsinized, uninfected human embryonic kidney cells failed to remove, or lower the titer of, the HSV 1 and HSV 2 nonvirion antibodies. All of these data taken together are interpreted as indicating that HSV 1 and HSV 2 play an etiologic role in certain human cancers, because they provide the kind of evidence by which virus-free experimental cancers can be proved to have been originally induced by such DNA viruses as polyoma, Simian Virus 40, or certain types of adenovirus.