The interaction of the TCR with the immunogenic peptide bound to MHC class II molecules leads to the activation of CD4⁺ T helper cells. T helper cells have been divided into two subsets, T_h1 and T_h2, on the basis of the cytokines secreted by them. T_h1 cells which secrete IL-2, IFN-γ and tumor necrosis factor-β induce delayed-type hypersensitivity, while T_h2 cells secreting IL-4, IL-5, IL-6 and IL-10 induce humoral immune response. However, the mechanism of selective activation of T_h1 and T_h2 cells in response to different antigens is not fully understood. In this study we examined the selective activation of T_h1 and T_h2 cells in response to strongly immunogenic synthetic peptides EYK(EYA)₃, abbreviated as K3; EYK(EYA)₄, abbreviated as K4; and EYKEYAAYA(EYA)₂, abbreviated as K1A2. These peptides are recognized by H-2^d T cells in the context of I-A^d, and are strongly cross-reactive in both T cell response and antibody response. The peptide K1A2 has very high affinity for I-A^d while K3 has a much lower affinity. K4 has affinity intermediate between K1A2 and K3. The peptide K1A2 induced T_h1 and K3 induced T_h2 cells in BALB/c mice as suggested by their cytokine profiles. K4 induced both T_h1- and T_h2-type cytokines. This was also confirmed by the analysis of peptide-specific antibody responses. K3 induced primarily lgG1 response, whereas K4 induced both lgG1 and lgG2a responses in vivo. There was a shift toward a T_h1-type cytokine profile when K3-primed T cells were challenged with K1A2 in vitro but K1A2-primed cells did not show any shift when challenged with K3. Immunization with higher doses of K3 shifted the response towards T_h1 type, while immunization with lower doses of K1A2 did not shift the response toward T_h2. We conclude that cells primed with high-affinity peptide are committed to differentiate into T_h1 irrespective of the priming dose and affinity of challenge antigen. On the other hand, the differentiation of cells primed with low-affinity peptide depends upon the dose of immunization and binding affinity of the challenge antigen for MHC.