The epitopes of 10 monoclonal antibodies (MAbs) directed against the phosphoprotein (P) of human respiratory syncytial (RS) virus (Long strain) were mapped onto the protein primary structure by testing their reactivity with both protein fragments and synthetic peptides. Seven epitopes were clustered in the N-terminal end, two others were located in the central region, and one epitope was mapped in the C-terminal end of the P molecule. The location of epitopes in the P protein sequence correlated with other properties, such as antigen binding competition and immunofluorescence staining of RS virus-infected cells. The antibodies that recognized epitopes of the N-terminal end revealed the presence of cytoplasmic inclusions while others gave mainly a diffuse cytoplasmic staining. Thus, accessibility of certain P protein epitopes seems influenced by the incorporation of this molecule to the inclusions of RS virus-infected cells.