Only M2e is accessible to Ab on fixed non-permeabilized HeLa-M2 cells. A. Control naive mouse serum (NMS), a mouse serum specific for the intracytoplasmic C terminus of M2 and M2e-specific MAb were tested for reaction with fixed HeLa-M2 and HeLa-C10 cells with (filled bars) and without (open bars) permeabilization. Mean Δ ODs of duplicates are shown. B. Fixed HeLa-M2 and HeLa-C10 immunosorbents were treated with increasing concentrations of trypsin (dilution of 2.5% is indicated) and then tested for reaction with a constant dose of M2e-specific MAb (triangles) or pooled serum from infection-immunized mice (squares). Symbols show Δ ODs (mean ± SEM of triplicates). One of two independent assays is shown.