EXPOSURE to various pathogens can stimulate at least two patterns of cytokine production by CD4-positive T cells(1-4) Responses that result in secretion of interferon-gamma (IFN-gamma), lymphotoxin and interleukin-2 (IL-2) are classified as T-helper-1 (Th1)(5,6); CD4(+) T-cell production of IL-4, IL-5, IL-9, IL-10 and IL-13 is called a T-helper-2 response (Th2)(5,6). Differentiation of CD4(+) T cells into either Th1 or Th2 cells is influenced by the cytokine milieu in which the initial antigen priming occurs(7-9). Here we use flow cytometry to identify the presence of intracellular cytokines (cytoflow) and analyse T-cell production of IFN-gamma and IL-4 from mice infected with Listeria monocytogenes or Nippostrongylus brasiliensis. We show that T cells bearing ya receptors discriminate early in infection between these two pathogens by producing cytokines associated with the appropriate T-helper response. Our results demonstrate that gamma delta T cells are involved in establishing primary immune responses.