The dose of foreign antigen can influence whether a cell-mediated or humoral class of immune response is elicited, and this may be largely accounted for by the development of CD4+ T helper cells (Th) producing distinct sets of cytokines. The ability of antigen dose to direct the development of a Th1 or Th2 phenotype from naive CD4+ T cells, however, has not been demonstrated. In this report, we show that the antigen dose used in primary cultures could directly affect Th phenotype development from naive DO11.10 TCR-αβ-transgenic CD4+ T cells when dendritic cells or activated B cells were used as the antigen-presenting cells. Consistent with our previous findings, midrange peptide doses (0.3-0.6 μM) directed the development of Th0/Th1-like cells, which produced moderate amounts of interferon γ (IFN- γ). As the peptide dose was increased, development of Th1-like cells producing increased amounts of IFN-γ was initially observed. At very high (>10 μM) and very low (<0.05 μM) doses of antigenic peptide, however, a dramatic switch to development of Th2-like cells that produced increasing amounts of interleukin 4 (IL-4) and diminishing levels of IFN-γ was observed. This was true even when highly purified naive, high buoyant density CD4+ LECAM-1(hi) T cells were used, ruling out a possible contribution from contaminating 'memory' phenotype CD4+ T cells. Neutralizing anti-IL-4 antibodies completely inhibited the development of this Th2-like phenotype at both high and low antigen doses, demonstrating a requirement for endogenous IL-4. Our findings suggest that the antigen dose may affect the levels of endogenous cytokines such as IL-4 in primary cultures, resulting in the development of distinct Th cell phenotypes.