Human serum and plasma block the binding of J14 antibodies but not hexavalent antibodies to the surface of GAS. After blocking non-specific sites on the surface of GAS with PBS-tween plus 5% BSA, normal human serum (▤), human plasma (▧) or PBS (□) were then incubated with the bacteria. Hexavalent (serum #2, table 1) and J14 (titer=25,600 against J14) rabbit sera (1:200) were then added to the adherent bacteria, incubated, washed and followed with HRP-goat anti-rabbit IgG. After washing, ABTS (KPL, Gaithersburg, MD) was used to quantify detectable rabbit hexavalent and J14 antibodies.