We investigated the ability of human recombinant interleukin-7 (IL-7) to enhance cytotoxic T lymphocyte (CTL) activity in vivo using mice infected with herpes simplex virus type-1 (HSV-1). IL-7 or interleukin-2 (IL-2) was administered twice daily to immune naive mice subjected to adoptive transfer of immune T cells after infection with HSV-1. The immunotherapeutic effect was measured by detecting the virus recovered from pinna. Administration of HSV-1 immune T cells to naive mice significantly increased their ability to clear the virus. Twice-daily injections of IL-7 at 200 IU provided an additional 20-fold reduction in virus load, compared with T cell therapy alone (P < 0.0005). Combining IL-2 and T cell therapy provided about a sevenfold reduction compared with T cell therapy alone (P < 0.0009). IL-7 also enhanced the antiviral effects of T cell therapy against HSV-1 through the enhancement of CD8+ CTLs, as observed with IL-2. These results indicate that IL-7 may be used adjunct to adoptive T lymphocyte therapy in modulating human viral diseases and cancer through enhanced immune T cell activities.