Effect of serum concentration on the HeLa-M2 ELISA. Preparations of MAb 14C2-S1-4 at 0.1 μg/ml in diluents containing various concentrations (0, 5, 10, 20%, A-D) of NMS were tested in ELISA for binding to HeLa-M2 (filled circle) and HeLa-C10 (open circle). The difference in OD (Δ OD) between HeLa-M2 and HeLa-C10 is plotted in red. Δ OD was subsequently used to estimate the concentration of M2e-specific Ab in serum-containing preparations. The symbols show means of triplicates ± SEM. A representative example of several independent experiments is shown, in which different NMS samples and MAb concentrations were tested.