The soluble form of the human respiratory syncytial virus (HRSV) attachment protein (Gs) was purified from the supernatant of infected cell cultures by immunoaffinity chromatography. Digestion of Gs with proteases and Western blot analysis identified two fragments that were partially resistant to protease degradation. Reactivity with diagnostic monoclonal antibodies located these two fragments in the primary structure of the G molecule. The large fragment spanned the C-terminal third of the G protein whereas the small fragment represented the N-terminal half of the large fragment. Purification of Gs from infected cells (either HEp-2 or M6) followed by protease digestion located host-cell-dependent glycosylation of the G protein in the unique part of the large protease-resistant fragment. The use of HRSV mutants encoding truncated G proteins allowed us to place some of the host-cell-dependent glycosylation differences in a small segment of the G protein. Interestingly, cell-specific glycosylations in the C-terminal half of the large protease-resistant fragment influenced the expression of certain epitopes located in its N-terminal half. These results bear important implications for the three-dimensional structure of the G glycoprotein.