Summary Vesicular stomatitis virus was disrupted by 0.1% SDS into RNA, lipid and the individual virus polypeptides. If the virus was first treated with 0.4% formaldehyde, the particles retained their bullet shape on treatment with 0.1% SDS, although they were extensively penetrated by phosphotungstic acid. Radiochemical analysis using virus labelled with [14C]-amino acids or [14C]-choline or [3H]-glucosamine indicated that about 82% of the glycoprotein and about 96% of the protein was retained in the formaldehyde pre-treated particles, which were then treated with SDS, although more than 90% of the lipid was lost. With Sinbis virus also the overall structure of the particles was retained when the virus which had been pre-treated with formaldehyde was treated with 0.1% SDS. With this virus, about 60% of the glycoprotein was retained in the SDS-treated formalinized particles despite the loss of all the lipid. These results suggest that the surface projections are located in close proximity to the core protein.