Sample graph of neutralization assay results with human reference sera. Virus neutralizing graphs by polyclonal anti-human reference sera (high, medium and low control)  obtained through the NIH Biodefense and Emerging Infections Research Resources Repository, NIAID, NIH in the absence (A) or presence (B) of 10% guinea pig serum. As negative control, control serum from naive cotton rats was included because of the lack of availability of negative human reference serum. Plaque reduction titers were calculated by regression analysis of the inverse dilution of serum that provided a 60% plaque reduction titer compared to control wells in the absence of serum. Results are means ± SD (triplicate). Settings of fluorescence reader are as described for Figure 3.