KRH-1636 is an orally active, selective and extremely potent CXC chemokine receptor 4 antagonist. KRH-1636 exhibits a potent and selective anti-HIV-1 activity. KRH-1636 efficiently blocked replication of various T cell line-tropic (X4) HIV type 1 (HIV-1) in MT-4 cells and peripheral blood mononuclear cells through the inhibition of viral entry and membrane fusion via the CXC chemokine receptor (CXCR)4 coreceptor but not via CC chemokine receptor 5. KRH-1636 also inhibits binding of the CXC chemokine, stromal cell-derived factor 1alpha, to CXCR4 specifically and subsequent signal transduction. KRH-1636 prevented monoclonal antibodies from binding to CXCR4 without down-modulation of the coreceptor. KRH-1636 seems to be a promising agent for the treatment of HIV-1 infection.