Competition of Ab binding to HeLa-M2 by free M2e peptide. Increasing concentrations of M2e(2–24) peptide were tested for ability to inhibit the binding of a constant dose of MAbs 14C2-S1-4 (triangles pointing up) and M2-1 (triangles pointing down) or pooled sera from mice immunized by three consecutive infections (squares) or three injections of (4)M2e-MAP (circles). MAbs were tested at 1 μg/ml and sera at 1/200 dilution. Ab binding in the presence of competing M2e peptide was expressed as percent of its binding (Δ OD) in the absence of competing peptide. The results show the composite of three independent assays, each performed in triplicate. Each symbol shows the mean ± SEM of nine replicates.