Lipopolysaccharide (LPS) potently stimulates human immunodeficiency virus type 1 (HIV-1) long terminal repeat-directed transcription in transfected monocyte-macrophage cell lines and dramatically increases HIV-1 production in the latently infected monocyte-macrophage-like cell line U1. This response to LPS, however, can only be observed after pretreatment of the U1 cells with granulocyte-macrophage colony-stimulating factor (GM-CSF). CD14, the differentiation antigen that acts as a receptor for complexes of LPS and LPS-binding protein, is now demonstrated to be involved in LPS-induced stimulation of HIV-1 replication. CD14 is shown to be expressed on a subpopulation of U1 cells only after treatment with GM-CSF and correlates with HIV-1 production stimulated by LPS. Importantly, only those U1 cells that express CD14 can be induced by LPS to upregulate HIV-1 production. In addition, a monoclonal antibody directed against CD14 can block LPS-induced stimulation of HIV-1 production from these latently infected cells.