The brains of 43 patients, some with various neurological disorders, other controls, were examined for herpes simplex virus (HSV) antigen using immunoperoxidase technique. The three patients with herpes simplex encephalitis shared a pattern of staining, consistent with that reported previously. However, of the other 40 patients, only two (one a patient with Alzheimer's disease, the other a control patient) showed areas of brain positive for HSV antigen (VA). In the patient with Alzheimer's disease VA was present within nerve and glial cells of the amygdala, within oligodendrocytes of the optic and olfactory tracts and in macrophages within the temporal cortex hippocampus and cerebellum. In the control patient VA was seen only in oligodendrocytes of optic chiasma and olfactory tract. The scarcity of these findings suggests "coincidental disease" processes within these two patients and means that any hypothesis implicating HSV as an aetiological agent in degenerative disease must still remain extremely speculative.