Previous reports have demonstrated that the U1 cell line, a model for postintegration latency, is defective at the level of Tat function and can be rescued by exogenously provided Tat protein. Sequence analysis of tat cDNAs from the U1 cell line identified two distinct forms of Tat, in agreement with the fact that this cell line contains two integrated human immunodeficiency (HIV) proviruses. One Tat cDNA lacked an ATG initiation codon, while the other contained an H-to-L mutation at amino acid 13 (H₁₃→L). Bothtat cDNAs were defective in terms of transcriptional activation of long terminal repeat-luciferase reporter gene in transient-transfection experiments. Introduction of the H₁₃→L mutation in a wild-type tat background caused a severe reduction in transcriptional activation. Introduction of the same mutation in an infectious HIV molecular clone caused a severely defective phenotype which could be rescued when the HIV proviral DNA was transfected in a Jurkat cell line stably expressing the Tat protein (Jurkat-Tat) or in Jurkat cells treated with tumor necrosis factor alpha. Infectious virus stocks generated in Jurkat-Tat cells were used to infect Jurkat cells and exhibited severely impaired growth which could also be rescued by infecting Jurkat-Tat cells. These observations define tat mutations as a mechanism for HIV postintegration latency.