Supernatants derived from peripheral blood mononuclear cell cultures of certain patients with the acquired immunodeficiency syndrome (AIDS) or its prodromes have the capacity to block T cell-dependent immune reactivity in vitro. T cells derived from a patient positive for antibody to the lymphadenopathy associated virus ( LAV ), and elaborating high titers of these soluble suppressor factors, were fused to a mutagenized clone of the human T lymphoblastoid cell line KE37 . Molecules capable of profoundly depressing T cell-dependent polyclonal antibody production and DNA synthetic responses, either directly or after incubation with normal adherent cells, were isolated from stable hybrid clones.