Depending on the microenvironment, murine gamma delta T cells differentiate into either Th1 (IFN-gamma-producing) or Th2 (IL-4-producing) cells. It is unclear, however, whether circulating human peripheral blood gamma delta T cells can be driven into Th1 or Th2 cells by modulation of the priming cytokine milieu. In this study, peripheral blood gamma delta T cells were stimulated by phosphoantigen (isopentenyl pyrophosphate) or Daudi lymphoma cells in the presence of Th1-priming (rIL-12, anti-IL-4 Ab) or Th2-priming (rIL-4, anti-IL-12 Ab) conditions. Single-cell analysis of cytokine secretion (IFN-gamma and IL-4) was performed by flow cytometry after 18 h and after restimulation of expanded gamma delta T cells. The early activation of resting gamma delta T cells was characterized by the induction of IFN-gamma. Priming under Th1 conditions induced a Th1 profile characterized by increased secretion of IFN-gamma and TNF-alpha, while Th2 conditions caused increased production of IL-4 (Th2 profile) by the gamma delta T cells. These results indicate that the major subset of human gamma delta T cells can be polarized into either Th1 or Th2 cytokine pattern depending on the cytokine milieu in which contact with antigen occurs.