gp39, a cytokine expressed on the surface of activated T cells, is essential for T cell-dependent antibody responses in vivo. We cloned and sequenced 1.2 kilobases of the 5' flank region of the human gp39 gene promoter and determined its transcription start site. When used in reporter gene assays, this DNA segment conferred promoter activity in response to T cell activation. gp39 promoter function in transfectants was inhibited by cyclosporin A, as is expression of the endogenous gp39 gene in T-lineage cells. At least 0.5 kilobase of the 5' flank region was required for promoter activity. Two putative binding sites for the NF-AT family of transcriptional activator proteins were identified at -259 to -265 and -62 to -69 with respect to the transcription start site. Both sites contributed significantly and independently to promoter activity in response to T cell activation. Additionally, when incubated in vitro with nuclear protein purified from activated human CD4 T cells, both of these sites preferentially bound the NF- AT family member, NF-ATp. These results suggest that NF-ATp, via binding to at least two cis-elements, is essential for the induction of gp39 gene expression in response to T cell activation.