In order to investigate further possible structural differences among the two subgroups of respiratory syncytial virus (RSV), we analysed the antigenic characteristics and size of structural proteins of 20 subgroup A and 43 subgroup B strains by their reactions with monoclonal antibodies (MAbs) directed against the proteins of RSV using immunofluorescence, ELISA and radioimmunoprecipitation assays. The latter test also enabled determination of the size of different structural components. The 37 MAbs employed were generated by immunization with both subgroup A and B strains. They represented specificities for distinct epitopes on five different structural proteins. The subgroup A strains proved to be relatively uniform. The fusion (F) protein, nucleoprotein (NP) and matrix (M) proteins of all strains tested had the same Mr and all except one strain had a phosphoprotein (P protein) of the same Mr. The F and P proteins were lower in Mr in B strains compared to A strains, which confirmed previous findings. The Mr of the large surface glycoprotein (G protein) of subgroup A strains varied slightly, probably on the basis of differing glycosylation. By contrast, the subgroup B strains exhibited substantial variation in the Mr of the G and also the P proteins and in reactivity with MAbs directed against the G and F proteins. Three size classes of the P protein were identified in B strains: 33K to 34K, 32K to 33K, and 31K to 32K. Twenty-seven subgroup B strains failed to react with four anti-G MAbs representing a single epitope, G2; the remaining 16 strains reacted with these MAbs. We designated these two sets of variants of B strains B1, which lacked the epitope, and B2, which had the epitope. The B1 strains also varied in the size of the G and P proteins. In contrast, all B2 strains had large G proteins and all except two strains had relatively large P proteins (33K to 34K). All subgroup B1 and B2 strains exhibited the same sizes of NP, F and M proteins. We conclude that the subgroup B strains of RSV include two variants, B1 and B2, and that the major difference between them resides in the G and P proteins.