One of the causes of genital tract infections in humans are herpes simplex virus types 1 and 2 (HSV-1, HSV-2). Although primary and recurrent infections can be clinically apparent and in part very serious, many infections are asymptomatic and result only in temporary genital shedding of virus (recurrences). During our investigations of vaginitis, strain IES of HSV-1 produced an asymptomatic infection. Replication in the murine vaginal (vag.) epithelium as well as antibody formation after vag. infection was comparable to those of survivors after infection with highly virulent strains. Titration of liver, spleen, ovaries, adrenal glands spinal cord, or brain after vag. IES infection revealed no virus, whereas after i.p. infection virus could be demonstrated in many organs examined. Histological examination with a DNA probe (in situ hybridisation), HSV antibodies (immunohistochemistry), and haematoxylin and eosin (HE) staining showed only small focal HSV lesions of the vaginal epithelium in early stages of the infection, never exceeding to the subepithelial tissue. Severe infiltrations and ulcerations after infection with highly virulent strains (17syn +, ER-) could never be demonstrated after IES vag. infection. Identical replication rates of both groups of HSV despite much greater areas of epithelial necrosis with the virulent strains may be explained by the large number of virus inactivating granulocytes induced by the virulent strains, thus inactivating the hypothetical higher virus load.