Herpes simplex virus 1 (HSV1) ICP34.5 deletion mutants are avirulent upon inoculation of both the central and peripheral nervous systems of mice, but they replicate to near wild-type titres in a number of non-neuronally derived cell lines in culture. Thus these mutants might be suitable for development as safe vectors for gene transfer to the nervous system. However, the mechanism of this avirulent phenotype in neuronal cells is at present poorly understood, although it has been suggested that nonpermissive cells infected with these mutants may undergo apoptosis, the function of ICP34.5 being to prevent this response and to allow continued virus replication. If this were the case ICP34.5 null mutants might be unsuitable for gene transfer as infected cells would quickly die, limiting the expression of a transgene. Here we have inserted a beta-galactosidase marker gene into a nonessential gene of an HSV1 strain 17+ mutant in which ICP34.5 has been deleted and also into a second mutant in which the virion transactivator protein VMW65 is also inactive. While all the mutants grew to high titre in tissue culture, mice inoculated by the foot-pad or intracranial route at high titre remained healthy until the end of the experiment. Moreover, beta-galactosidase was expressed either in the brain or in the dorsal root ganglia, depending on the site of inoculation. This suggests that in vivo the absence of ICP34.5 does not prevent the expression of a transgene in neuronal tissue and indicates that non-neurovirulent mutants lacking this gene may be suitable for further development as safe vectors for gene therapy in vivo.