Our present study provides evidence that the 4‐1BB signal is critical to CD28 co‐stimulation in maintaining T cell activation when CD28 has been down‐regulated because of repeated stimulation. The 4‐1BB signal synergized with CD28 co‐stimulation by lowering the threshold of anti‐CD28 required to sustain proliferation and IL‐2 production. The 4‐1BB signal also modulated CD28‐mediated cytokine profiles by markedly enhancing Th1 but suppressing Th2‐type cytokine production. The 4‐1BB signal generated Th1‐type cells, as identified by intracellular IFN‐γ production. IFN‐γ induction was detected preferentially in 4‐1BB‐expressing cells, but not in those expressing CD30. 4‐1BB and CD30 were induced in both CD4⁺ and CD8⁺ cells, but the location of the two molecules was mutually exclusive in each T cell subset. Our study suggests that the 4‐1BB signal regulates CD28 co‐stimulation in the targeted subset cells to favor Th1 development and maintain long‐term cell growth.