The Herpes Simplex Virus-1 (HSV-1) amplicon system is one of several viral-based strategies currently being developed for gene delivery into mammalian neurons for experimental or therapeutic purposes. Amplicon-containing viruses contain no HSV-1 genes and are amplified in titer relative to the helper viruses used to package them. In this way, they are designed to have a minimal impact on the physiology of transduced neurons. We show here, however, that amplicon preparations made using the 5dl1.2 helper virus selectively suppress sodium currents in cultured neurons by approximately 80% within 2 days of transduction and reduce average spike frequency in response to depolarization from 23 +/- 4 to 0.4 +/- 0.4 Hz. We observe similar suppression of Na(+) currents in cells treated with the 5dl1.2 helper virus alone, indicating that the helper virus retains the ability of wild-type HSV-1 to inhibit these currents potently. Staining amplicon-transduced neurons with anti-HSV antibodies, we find that 80% of the neurons express viral proteins, indicating that helper virus typically co-infects these cells. We conclude that Na(+) current suppression by the amplicon preparation results from the preferential coinfection of transduced neurons by the 5dl1.2 helper virus.