Superinfection by R5 HIV-1 infectious supernatant rescues X4 HIV-1 replicative capacity. After supernatant exchange from R5- and X4-infected cell cultures, the coreceptor use of the replicating viruses was tested at peak levels of virus replication onto U87-CD4-CCR5 and U87-CD4-CXCR4 cell lines. The outcome of these infections is represented in the embedded table showing a progressive rescue of X4 replicative capacity. X4s.i.w.R5 indicates X4-infected cells superinfected with R5 virus; and R5s.i.w.X4, R5-infected cells superinfected with X4 virus (n = 3).