Our experiments on the stepwise degradation of vesicular stomatitis virus into well characterized structural components have enabled us to relate some of the biological properties of the virus to certain of its structural units (Cartwright, Smale & Brown, 1969, 1970). For example, incubating the virus with trypsin produces a bullet-shaped structure similar to the virus except that it no longer possesses the surface projections. Unlike the virus, the projection-free component has low infectivity and does not produce neutralizing antibodies when inoculated into guinea-pigs, thus providing decisive evidence that the immunizing activity of the virus is associated with the surface projections.