Cultures of hamster embryo fibroblasts originally trans formed in vitro by herpes simplex virus (HSV) type 2 previously irradiated with ultraviolet light were resistant to superinfection by HSV-1 and HSV-2. The degree of resistance was dependent upon the concentration of the superinfecting virus. At an MOI of 1.0 infectious virus particle per cell, significant virus replication occurred. At an MOI of 0.001 per cell, little or no virus replication was detected. In normal hamster embryo fibroblasts, virus replicated when either high or low MOI were used. Early events in the virus-replicative cycle were found to proceed normally. The results suggest that HSV can initiate and complete the infectious cycle in a limited number of herpes-transformed cells, a finding similar to that observed for Epstein-Barr herpesvirus systems.