The development of mouse mAbs against different time-ordered GII.4 VLPs has greatly facilitated understanding of the complex antigenic relations between these strains by clearly demonstrating antigenic variation over time and by epidemic strain [34] . However, understanding of the human anti-GII.4 norovirus antibody response is essential not only for understanding the complex relationships between host immunity and virus antigenic change, but also for rational vaccine design based on defined neutralizing epitopes. Therefore, we developed a panel of human anti-GII.4 norovirus monoclonal antibodies to begin to characterize GII.4 antibody reactivity in the native virus host under natural infection conditions, noting that the norovirus preexposure histories in human volunteers are unknown and can only be inferred by human sera cross-reactive antibody binding and blockade patterns using time-ordered VLPs representing different outbreak and pandemic strains (Table 1) [13, 33] . Plasma and PBMC samples from 63 healthy individuals were collected in early 2009 and plasma binding titers (ED50) were measured by EIA against a panel of 6 different norovirus VLPs representing GI and GII strains ( Figure 1 ). The majority of tested samples reacted by EIA with variable ED50 titers to the panel of VLPs tested ( Figure S1 ). One sample (Donor NVB) was shown to react strongly with GII VLPs and was therefore chosen for further character- Figure 3 , and Figure S2 ). Significantly more antibody was needed to block GII.4.1997-PGM binding (EC50 0.4152 mg/ml) than GII.4.1987-PGM binding (EC50 0.3414 mg/ml) ( Figure 3B ) (p,0.05), supporting the hypothesis that subtle antigenic differences exist between these strains. 