Sera obtained from 15 patients with cervical cancer, 10 patients with breast cancer, and 15 control women, individually matched with the cervical c.ancer patients, were examined for antibodies to early proteins synthesized in herpes simplex virus type 2 (HSY-2)-infected cells. The method used was an indirect radioimmune precipitation test followed by polyacrylamide gel electrophoretic analysis of immune precipitates. The relative reactivity to a major early nonstructural protein (YPI34) was used to compare these selected sera. The results obtained suggest that cervical cancer patients possess sera with a higher reactivity to YP134 than breast cancer patients or matched healthy women, and that serum reactivity is independent of the level of neutralizing antibodies to HSY-2.