The mobility of the lipids in the bilayer of the envelope of vesicular stomatitis virus has been probed over its complete space by the biosynthetic incorporation of [N-13CH3]- choline as a probe for the polar head groups and [3-13C]- and [11-13C] oleic acid and [16-13C]- palmitic acid for the hydrophobic region of the bilayer. These precursors were effectively incorporated as established by the concomitant administration of the same precursors in radioactive form. Spin lattice relaxation time measurements (T1) of the 13C enriched segments in complete virus envelope allowed estimation of their mobility. The mobility of the polar head groups is restricted, probably due to ionic interactions with neighbouring acidic phospholipids (phosphatidylserine) and/or acidic side chains of the glycoprotein (G-protein). The rigidity of the hydrophobic part of the bilayer is due to the high cholesterol content and interaction with the immersing polypeptide chains of the G- and possibly M-protein. The rigidity is limited to a depth of about 15 A ranging from the inner and outer surface, whereas the inner core of the bilayer is fluid. Tryptic cleavage of the hydrophilic part of the G-protein allows the lipophilic immersing polypeptide fragment to enter further the bilayer which then reduces the fluidity of the hydrocarbon chains in the core region by lipid-protein interactions.