Neutralization susceptibility of the JFH1-based Core-NS2 genotype 2a recombinant increased with virus density for the three neutralizing antibody-containing sera H06, AA, and SA3. The neutralization susceptibilities of ultracentrifugation fractions with different densities of original genotype 2a virus were examined using three sera, H06 (A), AA (B), and SA3 (C). Neutralization was carried out at four different serum dilutions as indicated in each graph and performed as described in Materials and Methods. Neutralization data are shown as the means of three replicates with the SEM related to the mean of 6 replicates of virus only.