P proteins from 5 strains of GII-4 viruses (98Y96C, 03Y02C, 05Y04C, 06Y06aC1, and 06Y06bC1) were captured onto 96-well plates coated with pooled rabbit antisera against noroviruses and then blocked with a pair of human antiserum samples for 1 h prior to incubation with Leb oligosaccharide. The amount of bound Leb was detected using HRP-avidin. The y axis indicates the serum dilution (blockade titer) as determined from optical density values. The white bars indicate the antiserum titer at which 50% of binding was blocked (BT50) in comparison to unblocked controls, and the black bars indicate the titer at which 90% of binding was blocked (BT90). The results are averaged from at least two independent experiments.