CD28 costimulation is essential for CD4(+) T cell proliferation, survival, interleukin 2 (IL-2) production and T helper type 2 development. To define the nature of the signals that may drive different T cell responses, we have done a structure-function analysis of the CD28 cytoplasmic tail in primary T cells. CD28-mediated T cell proliferation and IL-2 production did not require a particular cytoplasmic domain. In contrast, IL-4 production was driven by the cooperative activity of specific motifs within the CD28 cytoplasmic tail. Using a gene-complementation approach, we provide evidence that one component of this T helper type 2 differentiation signal was mediated by 3-phosphoinositide-dependent protein kinase 1. Thus, different mechanisms underlie the induction of distinct T cell functional responses by CD28.