A transgenic TCR adoptive transfer system was used to visualize Ag-specific T cell activation and cytokine expression in vivo. After s.c. injection of peptide in adjuvant the entire Ag-specific population up-regulated IL-2 receptor α-chain expression, underwent blast transformation, and developed a memory-surface phenotype. A minority of the Ag-specific T cells produced predominantly IL-2 mRNA and localized at the T cell/B cell junction in draining lymph nodes. In the secondary response, a mixed cytokine pattern of both Th1 and Th2 cytokines was demonstrated. When peptide was administered i.v. without adjuvant, 50% of the Ag-specific cells expressed IL-2, but the peak of expression occurred before IL-2 receptor α-chain up-regulation, and only a minority of the Ag-specific T cells underwent blast transformation.