The HIV-1 co-receptor CCR5 has been thought a relevant target for small interfering RNA (siRNA)-based therapeutics. However, recent findings suggest that siRNA can stimulate innate cytokine responses in mammals. All siRNA agents tested were able to down-regulate the expression of CCR5, albeit with different efficiency (51-74% down-regulation), block HIV-induced syncytia formation between HIV-1 BaL-infected and uninfected CD4(+) cells or block single-round HIV-1 infection as measured by a luciferase reporter assay (46-83% inhibition). Conversely, siRNA directed against CCR5 did not affect replication of a vesicular stomatitis virus (VSV) pseudotyped virus, suggesting that inhibition of HIV replication was specific to CCR5 down-regulation. However, two of four siRNA tested were able to induce the production of interleukin (IL) IL-6 (sixfold induction) and IL-8 (ninefold induction) but no interferon (IFN)-alpha, IFN-beta, IFN-gamma, tumour necrosis factor (TNF)-alpha, monocyte chemoattractant protein (MCP)-1, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, RANTES, IL-1beta, IL-10 or IL-12p70 cytokine induction was noted. In the absence of detectable IFN-alpha, IL-6 or IL-8 may represent markers of non-specific effects triggered by siRNA.