Herpes simplex virus type 1 (HSV-1) has a broad host range although in natural human infections the virus is neurotropic, establishing latent infections in sensory neurons where the viral DNA persists as an intact episome. The establishment of latency does not depend on viral replication functions, suggesting that infection of non-neuronal cells, including tissue of myogenic origin, by replication defective mutants may result in genome persistence in a similar episomal state. In this report a replication defective HSV-1 recombinant vector containing the beta-galactosidase reporter gene under transcriptional control of the strong human cytomegalovirus immediate-early gene promoter (HCMV IEp-lacZ) was used to infect muscle cells in vitro and in vivo. This replication defective mutant virus (d120), deleted for both copies of the essential immediate-early gene (ICP4) and thus incapable of expressing early and late viral genes, displayed highly reduced cytotoxicity in myogenic cells. This vector infected both myoblasts and myotubes in culture with transgene expression persisting for at least 8 days. The transduction efficiency in myotubes was similar to myoblasts at several multiplicities of infection (MOIs), suggesting that HSV could infect differentiated muscle fibers and that myoblast differentiation would neither prevent expression of the cellular receptor(s) for the virus nor inhibit viral penetration. Direct inoculation of mouse muscle fibers in vivo with 10(6) to 10(8) plaque forming units (p.f.u.) of vector was sufficient to transduce significant numbers of muscle fibers in newborn mice and some fibers in adult normal and mdx mice. These results suggest that recombinant HSV-1 vectors may be useful for gene transfer to muscle.