Epitope-specific CD8+ T lymphocytes may play an important role in controlling human immunodeficiency virus (HIV)/simian immunodeficiency virus replication. Unfortunately, standard cellular assays do not measure the antiviral efficacy (the ability to suppress virus replication) of CD8+ T lymphocytes. Certain epitope-specific CD8+ T lymphocytes may be better than others at suppressing viral replication. We compared the antiviral efficacy of two immunodominant CD8+ T lymphocyte responses--Tat(28-35)SL8 and Gag(181-189)CM9--by using a functional in vitro assay. Viral suppression by Tat-specific CD8+ T lymphocytes was consistently greater than that of Gag-specific CD8+ T lymphocytes. Such differences in antigen-specific CD8+-T-lymphocyte efficacy may be important for selecting CD8+ T lymphocyte epitopes for inclusion in future HIV vaccines.