Results of transmission experiments in chimp 331 (see Figure 1, experiment II). After demonstrating that valley phase, blip phase, and pre–ramp-up phase infusions (50 mL) from each of 5 plasma donors were not infectious in chimp X331, this animal was challenged with a ramp-up phase inoculum containing 340 000 total RNA copies in a volume of 50 mL. Chimp X331 was infected as indicated by the detection of HCV RNA from weeks 1 to 16, but then HCV RNA cleared; HCV Abs and cell-mediated immune responses to HCV were not detected and the ALT level was not elevated. By week 18, there was no residual evidence that this infection had occurred.