Replication experiments in vitro have been employed as a biological assay system to ascertain defectiveness and the expresion of viral interference for strains of wild mumps virus, hiph passage cell culture-adapted laboratory virus, and attenuated vaccine virus.The efficiency of viral replication (EOR) was determined by comparing the multiplicity of infection (MOI) with the multiplicity of viral production (MOP). Serial passage of partially purified, cloned laboratory virus demonstrated a striking increase in FOP during early cycles with subseouent decline---the classic Von Mapnus phenomenon of defective viral interference. The replication kinetics of wild, laboratory and vaccine virus were evaluated using regular stock and cloned pools of virus. The EOR for stock wild virus was 4-fold greater than stock vaccine virus and 300-fold greater than stock laboratory virus. The EOR for vaccine and laboratory virus was increased strikingly by double cloning, while such limiting dilution bemadsoption placuing techniques employed to eliminate defective virions had little impact upon the EOR of wild virus.Defective virions play an important role in the development of persistent, slow virus infections. Wild mumps virus seems unlikely to produce such a pathobiologic process, however there may be some concern about attenuated mumps virus vaccine.