Upon antigenic stimulation, precursor CD4⁺ helper T‐cells differentiate into two subsets of effector cells, Th1 and Th2. These two subpopulations are defined by the pattern of cytokine expression that distinguishes these differentiated cells from their precursors. We have used reporter transgenic mice here to show that, during differentiation of precursor T‐cells into effector Th1 or Th2 cells, high levels of preformed activator protein (AP)‐1 complexes are accumulated. However, upon stimulation, the preformed AP‐1 complexes in effector Th2 cells, but not in Th1 cells, are able to induce high levels of AP‐1 transcriptional activity. Furthermore, in contrast to precursor T‐cells, the induction of AP‐1 transcriptional activity is independent of calcium and co‐stimulatory signals in effector Th2 cells. This AP‐1 transcriptional activity appears to correlate with the presence of JunB complexes, which accumulate differentially in effector Th2 cells, but not in precursor CD4⁺ T‐cells or effector Th1 cells. Unlike precursor cells, the activation of AP‐1 does not appear to be mediated by c‐Jun N‐terminal kinase (JNK) in effector Th2 cells. These results indicate that during differentiation of T‐cells, and probably other cell types, the signal requirements for the AP‐1 transcription machinery are reprogrammed to enable the differentiated cells to perform their specialized functions.