Summary Four proteins, GP1, VGP48, GP26 and VPM27, are associated with the envelope of respiratory syncytial (RS) virus. The status of GP1 has been uncertain, because a cellular glycoprotein migrates at the same position when Laemmli's discontinuous buffer system is used for PAGE, and because BSC-1 cells infected with the RSN-2 strain of RS virus appear not to contain GP1. However, additional evidence suggests that GP1 is a viral structural protein. (i) It is removed from cells by trypsin, while the cellular glycoprotein is not; (ii) it is separated from the cellular glycoprotein when the infected cells are analysed by neutral SDS-PAGE; (iii) it is present in the purified RSN-2 strain of RS virus produced by BSC-1 cells; (iv) it is also present in the purified Long strain of RS virus produced by either human or monkey cells. When purified Long strain virus is analysed by PAGE under non-reducing conditions, the glycoproteins VGP48 and GP26 migrate together, and VPM27 separates into two proteins, which one-dimensional peptide mapping suggests are not different proteins. These observations suggest that VGP48 and GP26 exist in the virion as a single molecule joined by disulphide bonds, and so resemble a paramyxovirus fusion protein, and that probably there are two forms of VPM27 which differ in either position or number of disulphide bonds.