A hierarchy exists among 'focal' areas of HSV-1 antigen positive cells in SJL/J, A/J, and PL/J mice. 'Focal' areas of viral antigen positive cells in serial sections of the BST-CB derived from 3 mice of each strain on day 6, 9, 12, and 15 PI were examined. Serial sections were examined by IHC and the number of 'focal' areas of viral antigen in the BST and CB per slide determined. As one 'focal' area could be present in several serial sections, the greatest diameter of the lesion at a magnification of 400x for the BST and 200x for the CB was recorded. Representative 'focal' areas of HSV-1 antigen positive cells in the BST-CB of virus infected 10-12 week ♀ mice are shown. Representative 'focal' areas in the BST of (a) SJL/J, 9 days PI (x400) (c) A/J, 6 days PI (x400) and (e) PL/J, 6 days PI (x400) mice are shown. Representative 'focal' areas in the CB of (b) SJL/J, 12 days PI (x200) (d) A/J, 9 days PI (x200) and (f) PL/J, 6 days PI (x200) mice are also shown.