FIG. 1.

In vivo titration of primary isolate HIV-1₅₀₁₆. Chimpanzees 1197 and 941 were inoculated with the indicated doses of HIV-1₅₀₁₆ at the times marked by the arrows. The viral burden is expressed as RNA copies/milliliter of plasma as assessed by the nucleic acid sequence-based amplification technique (33). Titers of neutralizing antibody to HIV-1_MN, expressed as the reciprocal of the serum dilution at which 50% neutralization was observed, were assessed as previously described (31), with H9 cells as targets of infection.