Hapten (trinitrophenyl)-specific T cell lines were obtained by repeated stimulation of lymph node cells from immune mice with Ag in vitro. These T cell lines show phenotypic properties and a pattern of cytokine production typical of Th1 cells and consisted of more than 90% V beta 8.2+ T lymphocytes and 6 to 9% gamma/delta + T lymphocytes. The lines mediate a local passive transfer of DTH when injected at the site of Ag challenge but fail to mediate a systemic passive transfer of DTH when injected i.v. However, a successful systemic passive transfer of DTH was observed when IL-4 was given to recipient mice together with the T cell lines or when the T cell lines were incubated in vitro with IL-4. IL-4 enables systemic, specific passive transfer of DTH at a dose of 10 pg/ml in vitro and at a dose of 10 pg/mouse in vivo; it is effective when injected 4 h before cell transfer but not when given 1 to 5 days earlier. Cytofluorimetric analysis shows that the gamma/delta + cells and not the V beta 8.2+ cells of the line express IL-4R and a good systemic transfer of DTH is observed when gamma/delta + cells are incubated in vitro with IL-4 and then injected together with V beta 8.2+ cells into recipient mice. In contrast, injection of V beta 8.2+ cells treated with IL-4 together with gamma/delta + cells fails to transfer DTH. Overall, the present results show that IL-4 is an important mediator in the DTH reaction and that gamma/delta + cells are one of the targets of its action.