The differentiation of T cells along different lineages is central to the control of immunity. Here we have used a conditional gene knockout system to delete PKC λ/ι selectively in activated T cells. With this system we have demonstrated that PKCλ/ι is necessary for T-helper cell (Th2) cytokine production and optimal T-cell proliferation and allergic airway inflammation in vivo. Our data demonstrate that the activation of the transcription factors nuclear factor of activated T cells and NF-κB is impaired in PKCλ/ι-deficient activated T cells. In addition, we present genetic knockout evidence in ex vivo experiments with primary T cells that PKCλ/ι is critical for the control of cell polarity during T-cell activation. Therefore PKCλ/ι emerges as a critical regulator of Th 2 activation.