Alterations in the expression of CXCR4 and CCR5, the co-receptors for HIV entry, may be associated with susceptibility of monocytic cells to HIV infection. Interferon (IFN)-gamma has been shown to inhibit HIV replication in monocytic cells, but the molecular mechanism involved is not well understood. To determine if IFN-gamma regulates HIV replication by altering CXCR-4/CCR-5 expression and hence virus entry into monocytic cells, we investigated the effects of IFN-gamma on CXCR-4 and CCR-5 expression and its biological implications with respect to HIV entry, replication and chemotaxis towards the CXCR-4 and CCR-5 ligands SDF-1 and MIP-1alpha, respectively. IFN-gamma decreased CXCR-4 and CCR-5 expression on monocytes derived from HIV-negative adults, HIV-positive adults and HIV-negative cord blood. This down-regulation of chemokine receptor expression did not result in a corresponding change in mRNA expression but was associated with elevated levels of the endogenously produced chemokines SDF-1 and RANTES. Furthermore, IFN-gamma inhibited chemotaxis in response to SDF-1 and MIP-1alpha, inhibited HIV replication, but failed to inhibit virus entry in monocytic cells. These results suggest that although IFN-gamma-induced down-regulation of CXCR-4 and CCR-5 expression is associated with an inhibition of SDF-1-/MIP-1alpha-mediated chemotaxis, IFN-gamma-induced inhibition of HIV replication may be mediated at levels subsequent to the virus entry.