We investigated the effect of costimulation through CD28 and CD11a on the differentiation of human naive CD4+ T cells with restricted cytokine production profiles. Interleukin (IL)-4 and interferon-γ (IFN-γ) were measured by ELISA and IL-2 was detected by a bioassay. Naive CD4+ T cells proliferated and produced IL-2 upon cross-linking of CD3, and costimulation through CD28 enhanced IL-2 production. After repeated stimulation, CD4+ T cells which were stimulated in the absence of costimulation through CD28 lost their ability to secrete IL-2 and started secreting IL-4 and IFN-γ. Instead in the presence of costimulation through CD28, they secreted IL-2, IL-4 and IFN-γ. Blocking of endogenous IL-4 activity with anti-IL-4 Ab suppressed the IL-4 secretion and prolifeation of T cells.