HIV-1 proviral DNA contains two binding sites for the transcription factor NF-x B. HIV-1-infected individuals have, on average, abnormally high levels of tumour necrosis factor α (TNFα) and abnormally low plasma cysteine levels. We therefore investigated the effects of cysteine and related thiols on HIV-1 replication and NF-x B expression. The experiments in this report show that cysteine or N-acetylcysteine (NAC) raise the intracellular glutathione (GSH) level and inhibit HIV-1 replication in persistently infected Molt-4 and U937 cells. However, inhibition of HIV-1 replication appears not to be directly correlated with CSH levels. Cysteine and NAC also inhibit NF-x B activity as determined by electrophoretic mobility shift assays and chloramphenicol acetyl-transferase (CAT) gene expression under control of NF-x B binding sites in uninfected cells. This suggests that the cysteine deficiency in HIV-1-infected individuals may cause an over-expression of NF-x B-dependent genes and enhance HIV-1 replication. NAC may be considered for the treatment of HIV-1-infected individuals.