We have investigated methods for modulating immune responses, against herpes simplex virus (HSV), generated from DNA vaccination by co-delivery of genes encoding costimulatory molecules. A strong delayed-type hypersensitivity (DTH) reaction was induced in mice co-injected via the intradermal (i.d.) route with a eukaryotic expression plasmid encoding the CD80 molecule (pCD80) and a plasmid encoding the glycoprotein D of the HSV-2 (pgD). Furthermore, when spleen cells from these mice were cultured in the presence of inactivated HSV, a significant increase in the expression of interleukin-2 receptor (IL-2R) was observed in the CD4 subset compared with mice immunized only with pgD. Analysis of cytokine synthesis at the single-cell level indicated that CD80 genes induce a significant increase in the number of interferon-gamma (IFN-gamma)-, IL-2- and IL-4-secreting cells in the spleen. On the other hand, co-administration of the CD80 gene via the intramuscular (i.m.) route did not induce an increase in the cell-mediated immune response. When a plasmid carrying the CD86 gene (pCD86) was co-injected via the i.m. route with the pgD plasmid, a small decrease in the number of IFN-gamma-secreting cells was observed. This down-regulation of the immune response was also observed when eukaryotic expression cassettes for CD80 and for CD86 were co-administered with the pgD plasmid via the i.d. route. However, co-injection of pCD86 via the i.m. route produced a small increase in the number of IL-4-secreting cells. When immunized mice were challenged intravaginally with 100 plaque-forming units of virus, only co-injection of the CD80 gene by the i.d. route provoked an adjuvant effect compared with mice immunized with pgD alone. A reduction in the titres of HSV in vaginal washings was observed together with a decrease in the lesion score.