Our estimate of the rate of HIV-1–specific CTL lysis totalled across all CTL responses of 0.1 to 0.2 d−1 is considerably lower than in vitro estimates (1 d−1 [37,38]), possibly reflecting the artificial nature of experiments in which a high density of peptide-pulsed B cells or transformed CD4+ cells infected with a high multiplicity of infection are exposed to CD8+ cells in a homogenous environment. Here, CD4 is linked to infection.