Abstract: In addition to virions, herpesvirus-infected cells produce non-infectious particles, termed L-particles, which consist of tegument surrounded by envelope but lack the nucleocapsid. L-particles of a herpesvirus can be prepared substantially free of infectious virions, e.g. by allowing an appropriate temperature-sensitive mutant to replicate at its non-permissive temperature, and such L-particles are useful as a component of a vaccine against the herpesvirus. They can also be prepared to contain a foreign protein or peptide, by use of a recombinant herpesvirus.