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Human immunodeficiency virus-(HIV) infected monocyte-macrophages may contribute to the pathogenesis of HIV-associated immune deficiency and dysfunction by acting as a target and potential reservoir for the virus in vivo, and by functioning abnormally following infection. We have shown that HIV-infected macrophages fuse with uninfected CD4-expressing lymphoid cells in vitro; this may provide an additional mechanism for CD4 lymphocyte depletion in vivo. We report here the inhibition of syncytium formation between HIV-infected macrophages and uninfected CD4-expressing T-lymphoid cells by monoclonal antibody S3.5, directed against an epitope of CD4 involved in binding HIV gp120, by a recombinant protein that comprises the full-length extracellular domain of the CD4 molecule, and by recombinant full-length HIV envelope glycoprotein, gp120. These results indicate that both molecules (gp120 and CD4) are critical to the fusion process, and suggest that gp120 is expressed on the surface of HIV-infected monocyte-macrophages.