A number of eucaryotic viruses have devised strategies to minimize the deleterious effects on protein synthesis caused by activation of the interferon-induced, double-stranded-RNA-activated protein kinase, P68. In a recent report, we described the down regulation of the P68 protein kinase in cells infected by human immunodeficiency virus type 1 (HIV-1) (S. Roy, M. G. Katze, N. T. Parkin, I. Edery, A. G. Hovanessian, and N. Sonenberg, Science 247:1216-1219, (1990). We now present evidence that such a decrease in amounts of P68 could be essential for HIV-1 replication because of the presence of the Tat-responsive sequence (TAR sequence) present in the 5' untranslated region of HIV-1 mRNAs, which activates the P68 kinase. We found that poly(A)+ mRNAs prepared from HIV-1-infected cells efficiently activated the protein kinase as did mRNAs from stably transformed cell lines constitutively expressing the TAR region. Furthermore, we found that TAR-containing RNAs complexed with purified P68 protein kinase in vitro by two independent assays and could be cross-linked to P68 kinase present in a HeLa cell extract. Experiments using in vitro-synthesized wild-type and mutant TAR RNAs revealed that both the efficient binding to and the activation of P68 kinase were dependent on the TAR RNA stem structure. The TAR-P68 complex could be competed out by a synthetic RNA that bound to and activated the protein kinase but not by a synthetic RNA that bound with low affinity and did not activate P68. The possible biological consequences of a P68-TAR interaction that may include the switch from latent to active virus replication are discussed.