Clinical and immunological studies were carried out in 697 patients (913 eyes) for a more accurate diagnosis of herpetic involvement of the posterior segment of the eye by the focal allergic test (FAT) based on the delayed type hypersensitivity phenomenon. FAT consists in detection of the focal reaction in the eye, regularly developing after 2-3 intracutaneous injections of herpetic vaccine to types I and II herpes simplex virus in a dose of 0.2 ml every other day. The reaction is assessed by clinical and functional methods: visometry, campimetry, biomicroscopy, 9-point test, Amsler network, Hedinger's phenomenon, photocolor stress test, Rabkin's polychromatic tables, fluorescent angiography of the fundus oculi, electrophysiological methods, critical frequency of flash fusion, reflection microscopy of the corneal endothelium, and color campimetry realized on an IBM-compatible computer before and after vaccination of patients with suspected herpetic involvement of the eye. FAT was positive in 395 (56.6%) patients in 568 (62.2%) eyes. Examination of the threshold relative brightness sensitivity before and after FAT revealed positive results in 70% of patients. FAT is an accurate non-invasive method for the diagnosis of various manifestations of intraocular herpes. Relative safety of FAT recommends it for clinical practice to be used in the diagnosis of herpetic involvement of the posterior segment of the eye.