The sensitive peroxidase-antiperoxidase (PXAPX) method individually and in conjunction with the Papanicolaou (PAP) stain was used to detect herpes simplex virus (HSV) in specimens from human female genitalia. Initial studies using a model system of HSV-1or HSV-2-infected Vero cells established (i) acetone as the most effective fixative, (ii) optimal dilutions of preimmunization and anti-HSV serum for differentiation of infected from noninfected cells, (iii) optimal concentration of 3,3'-diaminobenzidine tetrahydrochloride and H202 for maximal staining of infected cells with minimal background reaction, and (iv) removal of endogenous peroxidase with absolute MeOH. These various parameters, once established, were utilized in the PXAPX or PXAPX-PAP on human specimens from the vulva or cervix. In these specimens, examined by standard light microscopy, PXAPX-positive cells were dark brown with a single nucleus or multiple nuclei. By coupling the PAP to the PXAPX, detailed nuclear observations of PXAPX-positive cells were possible and revealed nuclear changes consistent with HSV infection, including syncytium formation, chromatin condensation, and an occasional Cowdry type A inclusion. The PXAPX and PXAPX-PAP correlated (r = 0.742) over a period of 72 h with HSV isolation from these samples. MATERIALS AND METHODS Virus. Virus stocks of HSV-1 (strain 69-85) and HSV-2 (strain 316-D) were obtained from F.