The long terminal repeat (LTR) of the human immunodeficiency virus (HIV) contains three binding sites for the transcriptional factor Sp1. In order to investigate the role that the Sp1-binding sites play in regulation of HIV replication, we have introduced a deletion of all three Sp1-binding sites into the LTR of an infectious molecular clone of HIV. Viral stocks have been prepared from this mutant virus, designated dl-Sp, and these stocks have been used to study its replicative ability in human T cells. The dl-Sp virus replicated efficiently in MT4 cells and in phytohemagglutinin-stimulated human peripheral blood lymphocytes, but it replicated poorly and with delayed kinetics in A3.01 (CEM) T cells unless those cells had been treated with the cytokine tumor necrosis factor alpha. Gel retardation assays to study the levels of DNA-binding proteins present in these cells showed that NF-kappa B activity could be detected in the nuclei of MT4 cells but not in A3.01 cells unless they had been treated with tumor necrosis factor alpha. Thus, the presence of NF-kappa B activity appeared to be required for efficient replication of an HIV whose LTR Sp1-binding sites had been deleted. This suggests that NF-kappa B can functionally compensate for Sp1 in activating HIV replication. The HIV LTR is therefore similar to the promoter-enhancer units of other viruses in that it is composed of multiple functional elements that may contribute differently to viral replication depending on the levels of DNA-binding proteins present in the target cells.