The RSV-F protein obtained from Co-IP was further analyzed by LC–MS/MS. Mass spectrometry identified the RSV-F peptide TGWYT(+79.97)SVITIELS(+79.97)NIK, in which Thr54 (T54) and Ser62 (S62) were phosphorylated. The MS/MS fragment ion spectra were highly consistent with the phosphorylation site assignment.