Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) enter target cells by forming a complex between the viral envelope protein and two cell-surface membrane receptors : CD4 and a 7-span transmembrane chemokine receptor (reviewed in refs 1-3). Isolates of HIV that differ in cellular tropism use different subsets of chemokine receptors as entry cofactors : macrophage-tropic HIVs primarily use CCR5, whereas T-cell-tropic and dual-tropic isolates use CXCR4 (refs 1-3) receptors. HIV-mediated signal transduction through CCR5 is not required for efficient fusion and entry of HIV in vitro(4,5). Here we show that recombinant envelope proteins from macrophage-tropic HIV and SIV induce a signal through CCR5 on CD4(+) T cells and that envelope-mediated signal transduction through CCR5 induces chemotaxis of T cells. This chemotactic response may contribute to the pathogenesis of HIV in vivo by chemo-attracting activated CD4(+) cells to sites of viral replication(1,2). HIV-mediated signalling through CCR5 may also enhance viral replication in vivo by increasing the activation state of target cells. Alternatively, envelope-mediated CCR5 signal transduction may influence viral-associated cytopathicity or apoptosis.