The fact that all tested antibodies were not reactive in western blot analyses using extracellular HCMV particles as antigen indicated that the intact three-dimensional protein conformation was crucial for binding (data not shown). Therefore, we used indirect immunofluorescence of transiently expressed fragments of gB in Cos7 cells to obtain further information on the binding sites of the mabs. Two distinct reactivity patterns were observed for the mabs (Group A and Group B in Fig. 2 ). Full length gB 1-906 and fragments as short as gB 1-447 were reactive with the entire set of Table 1 . Epitope specificity and neutralizing activity of IgG HCMV-gB specific antibodies. (Fig. 2 ). For Group A mabs gB residues 100-342 were sufficient for binding whereas for Group B antibodies a larger fragment of gB comprising residues 100-448 was required for binding, indicating that the gB region between aa 100-448 harbors at least two distinguishable antibody target sites.