Dextran sulfate is a potent inhibitor of human immunodeficiency virus (HIV) binding and replication in lymphocytic cell lines. In this study, we demonstrate that the effect of dextran sulfate and heparin depends on the host cell type and on the V3 loop, the principal neutralizing determinant of HIV gp120. In particular, when dextran sulfate was tested on primary human macrophages infected with macrophage-tropic viruses, enhancement of infection was observed in 6 of 11 independent macrophage preparations and with 5 of 13 primary HIV isolates. Our in vitro observations might explain why enhanced HIV replication was observed in HIV-infected patients treated with dextran sulfate.