We have examined the antigen presenting cell (APC) requirements for primary T cell activation and T helper (Th) cell phenotype differentiation using naive CD4⁺ T cells from αβ TCR transgenic mice. Purified dendritic cells were the principal cell required for induction of primary ovalbumln peptide specific T cell activation and clonal expansion. However, dendritic cells did not induce differentiation of T cells toward Th1 or Th2 phenotype. Addition of IL-4 during primary dendritic cell stimulations of T cells resulted in the development of a Th2 phenotype which produced high levels of IL-4 during secondary and tertiary stimulation. In contrast, development of Th1 cells producing high levels of IFN-_γ could not be induced with dendritic cells alone but required the addition of appropriately activated macrophages. Addition of splenic or peritoneal B cells did not induce Th1 development. Activated splenic macrophages induced Th1 development via a non-MHC restricted mechanism. Thus, requirements for induction of proliferation of naive CD4⁺ T cells are distinct from those directing Th1 phenotype development. IL-12 could replace the requirement for macrophages to induce Th1 development when T cells were activated with dendritic cells. Furthermore, this IL-12 mediated development of Th1 cells producing high levels of IFN-_γ was dependent on IFN-_γ.