In a recent report, the neurovirulence of herpes simplex virus type 1 (HSV-1) was mapped to the ICP34.5 gene (J. Chou, E. R. Kern, R. J. Whitley, and B. Roizman, Science 250:1262-1266, 1990). In this report, specific mutations within ICP34.5 were constructed in HSV-1 strain 17syn+ to determine the effects of these mutations in a fully neurovirulent isolate. It was found that termination of the ICP34.5 gene after the N-terminal 30 amino acids resulted in a mutant, 17termA, which was 25- to 90-fold reduced in neurovirulence. This reduction of neurovirulence was associated with restricted replication of the mutant virus in mouse brain. The reduced replication phenotype was also evident in the trigeminal and dorsal root ganglia following inoculation at the periphery. 17termA was capable of replicating with wild-type kinetics in mouse footpads, and therefore the restriction seen in neural tissues was not due to a generalized replication defect in mouse cells. Significantly, replication of the mutant was also restricted in the mouse cornea in vivo and in confluent primary mouse embryo cells and mouse 10T1/2 cells in vitro. However, 17termA replicated with much greater efficiency in subconfluent mouse embryo cells, suggesting that the physiological state of the cell may be an important factor for productive replication of this mutant. Restoration of the ICP34.5 gene to the mutant resulted in a virus which displayed wild-type neurovirulence and replication kinetics in all cells and tissues tested.