Serial passage of undiluted herpes simplex virus types 1 and 2 resulted in cyclic production of infectious and defective virions. Defective virus production was characterized by the appearance of a new species of viral DNA with a higher bouyant density in CsCl than standard viral DNA. Measurement of the infectivity titer and DNA synthesis revealed that the defective particles interfered with the replication of standard virions and stimulated the overproduction of a large molecular weight (175,000 daltons) polypeptide.