Cells at near confluence were detached by a brief 0.025% trypsin/0.02% EDTA treatment. Viable cells, 2 × 105 per well of 96 culture microtiter plates, were incubated with primary anti-H type 2 or broad reactive anti-A monoclonal antibodies 19-0LE and 3-3A respectively, in PBS containing 0.1% gelatin for 30 min at 4°C (Bara et al. 1988; Mollicone et al. 1996). After three washes with this same buffer, a 30-min incubation with the secondary anti-mouse IgG FITC-labeled antibody (Sigma) was performed at 4°C. After washing, fluorescence analysis was performed on a FACSCalibur (Becton-Dikinson, Heidelberg, Germany). The expression of the EGFP-S protein construct was detected by its autofluorescence on the FL1 channel.