To study human immunodeficiency virus (HIV)–specific cellular immunity in vivo, we transferred syngeneic lymphocytes after ex vivo expansion and transduction with a chimeric receptor gene (CD4/CD3-ζ) between identical twins discordant for HIV infection. Single and multiple infusions of 10¹⁰ genetically modified CD8⁺ T cells resulted in peak fractions in the circulation of approximately 10⁴ to 10⁵modified cells/10⁶ mononuclear cells at 24 to 48 hours, followed by 2- to 3-log declines by 8 weeks. In an effort to provide longer high-level persistence of the transferred cells and possibly enhance anti-HIV activity, we administered a second series of infusions in which both CD4⁺ and CD8⁺ T cells were engineered to express the chimeric receptor and were costimulated ex vivo with beads coated with anti-CD3 and anti-CD28. Sustained fractions of approximately 10³ to 10⁴ modified cells/10⁶ total CD4⁺ or CD8⁺cells persisted for at least 1 year. Assessment of in vivo trafficking of the transferred cells by lymphoid tissue biopsies revealed the presence of modified cells in proportions equivalent to or below those in the circulation. The cell infusions were well tolerated and were not associated with substantive immunologic or virologic changes. Thus, adoptive transfer of genetically modified HIV-antigen–specific T cells was safe. Sustained survival in the circulation was achieved when modified CD4⁺ and CD8⁺ T cells were infused together after ex vivo costimulation, indicating the important role played by antigen-specific CD4⁺ T cells in providing “help” to cytotoxic effectors.