Defective herpes simplex virus type 1 (HSV-1) vectors can deliver genes into both mitotic and postmitotic cells, including neurons and these vectors, therefore, have great potential use. pHSVlac, the prototype HSV-1 vector, expresses the E. coli Lac Z gene from the HSV-1 immediate early 4/5 promoter. pHSVlac can stably express beta-galactosidase in a range of mammalian cell lines and in neurons from throughout the nervous system, both in culture and in the adult rat brain. Thus, HSV-1 vectors may be useful for studying HSV-1 latency, neuronal physiology, and performing gene therapy for neurological conditions. A virus stock of pHSVlac consists of identical HSV-1 particles containing either pHSVlac DNA or the HSV-1 helper virus DNA. Thus, a cell can be infected with the pHSVlac virus, the helper virus, or both; consequently, it is important to determine if the helper virus influences the behavior of pHSVlac. The effect of the helper virus on expression of beta-galactosidase from pHSVlac was investigated: it was demonstrated first, that pHSVlac can be efficiently packaged into HSV-1 virus particles using each of five HSV-1 temperature sensitive (ts) mutants as helper virus. Second, pHSVlac grown with each of the five HSV-1 ts mutants expressed high levels of beta-galactosidase. Third, pHSVlac grown with HSV-1 strain 17 ts K as helper virus expresses the same amount of beta-galactosidase in the absence or presence of ts K. Thus, pHSVlac can efficiently express a gene independent of the HSV-1 helper virus.