RANTES is a chemoattractant cytokine (chemokine) whose gene is expressed immediately after stimulation of several cell types but upregulated late (3 to 5 days) after activation in normal T lymphocytes. Here we describe two cis-acting elements in the human RANTES promoter that act in T lymphocytes. One site interacts with NFIL6, which is activated within the first 24 h after T-cell activation. The second site binds an apparently novel complex that is upregulated later, between days 3 and 5. These data provide an explanation for the immediate-early expression of RANTES in some cell types and identify apparently novel factors contributing to late RANTES transcription in T cells. The results reveal a developmental switch occurring during normal T-cell maturation coincident with the onset of terminal differentiation and the binding of late-acting factors to sequences of the RANTES promoter.