Results of transmission experiment I in chimp X355. Fifty milliliters of plasma from each of 5 donors in the pre–ramp-up phase of HCV infection were infused in a single experimental procedure into chimp X355 (see Figure 1, experiment I) and infectivity monitored by weekly or biweekly measurement of ALT level (solid line), HCV RNA (dashed line and box), anti-HCV (box), and cell-mediated immune response (bottom panel). Transient HCV infection is demonstrated. Note that HCV RNA was no longer detectable by week 10 and that Ab to HCV disappeared by week 18 after infusion. Cell-mediated immune responses, measured in a proliferation assay, peaked at week 4 and considerably diminished by week 10.