The susceptibility of phenotypically CCR5-negative resting CD4 T cells for membrane fusion with a CCR5-specific HIV-1 envelope was analysed using a novel sensitive fusion assay. A very low overall density of CCR5 on T cells expressing high levels of CD4 was shown to be sufficient for HIV envelope-mediated membrane fusion. These findings are relevant to the understanding of how HIV-1 R5 strains enter and replicate in resting CD4 T cells in vivo.