CD8⁺ T lymphocytes from individuals infected with human immunodeficiency virus–type 1 (HIV-1) secrete a soluble activity that suppresses infection by HIV-1. A protein associated with this activity was purified from the culture supernatant of an immortalized CD8⁺ T cell clone and identified as the β-chemokine macrophage-derived chemokine (MDC). MDC suppressed infection of CD8⁺ cell–depleted peripheral blood mononuclear cells by primary non–syncytium-inducing and syncytium-inducing isolates of HIV-1 and the T cell line–adapted isolate HIV-1_IIIB. MDC was expressed in activated, but not resting, peripheral blood mononuclear cells and binds a receptor on activated primary T cells. These observations indicate that β-chemokines are responsible for a major proportion of HIV-1–specific suppressor activity produced by primary T cells.