Abstract Naive CD4 + T cells differentiate into two types of helper T cells showing an interferon-γ-predominant (Th1) or an interleukin-4-predominant (Th2) cytokine secretion profile after repeated antigenic stimulation. Their differentiation can be influenced by slight differences in the interaction between the T cell receptor (TCR) and its ligand at the time of primary activation. However, the primary response of freshly isolated naive CD4 + T cells to altered TCR ligands is still unclear. Here, we investigated the primary response of splenic naive CD4 + T cells derived from transgenic mice expressing TCR specific for residues 323–339 of ovalbumin (OVA323–339) bound to I-A d molecules. Naive CD4 + T cells secreted either Th1- or Th2-type cytokines immediately after stimulation with OVA323–339 or its single amino acid-substituted analogs. Helper activity for antibody secretion by co-cultured resting B cells was also found in the primary response, accompanied by either low-level Th2-type cytokine secretion or no apparent cytokine secretion. Our results clearly indicate that dichotomy of the Th1/Th2 cytokine secretion profile can be elicited upon primary activation of naive CD4 + T cells. We also demonstrate that the helper activity of naive CD4 + T cells for antibody production does not correspond to the amounts of the relevant cytokines secreted.