We have used the organotypic culture system as a model to study the initial infectious process and spread of herpes simplex virus type 1 (HSV-1) in fully stratified and differentiated human epithelial tissue. The growth kinetics of HSV-1 were determined in organotypic tissues of human epidermal or ectocervical origin. Concurrently, we followed the spread of HSV-1 by immunostaining thin sections of infected organotypic tissue. After HSV-1 was applied to the top cornified epithelial layer, virus penetrated to the basal layer of replicating epithelium and grew to high titers. The virus was limited in its spread in that not all cells within the tissue had demonstrable infection. A ribonucleotide reductase mutant, ICP6 delta, could infect and replicate in basal layers of the organotypic tissues. However, we found that spread was limited in, and to, the basal cell layer. Peak ICP6 delta titers were 100-fold less than in cultures infected with wild-type HSV-1. Studies of HSV mutants should allow us to further define the role of specific viral genes which are associated with infection and spread in a tissue culture system that mimics the initial portal of entry for certain HSV infections.