CD28 is the primary T cell costimulatory receptor, and upon ligation with its ligands, it enhances T cell proliferation and IL-2 synthesis. In this study we examined the role of CD28 in the initial proliferative response and cell cycle entry of T lymphocytes. Stimulation through CD3 alone resulted in a poor proliferative response, while in the presence of CD28 costimulation a strong increase in the number of cells in S-phase could be detected after 48 h of stimulation. CD28 costimulation enhanced expression of cyclin D3 and induced down-regulation of p27kip1 expression. Cross-linking CD28 was much more effective in inducing cyclin D3 expression and in down-regulating p27kip1 expression than addition of IL-2. Blocking experiments, using antibodies that neutralize IL-2 or the IL-2 receptor, showed that the effects induced by CD28 are independent of endogenous IL-2. Moreover, using a variety of immunosuppressants that interfere with IL-2 signaling pathways, we were able to show that IL-2 is not required for cell cycle entry induced by CD28 costimulation. From these experiments it can be concluded that CD28 and IL-2 use different signaling pathways for down-regulation of p27kip1 expression. We hypothesize that costimulation through CD28 is responsible for initial cell cycle entry of T lymphocytes, while IL-2, which is produced after costimulation, might be involved in sustaining proliferation.