Nine measles vaccine preparations, including four different viral strains, provided by eight different manufacturers were analysed by Northern blot for the nature of their nucleocapsid RNAs. Out of nine preparations, six were shown to contain subgenomic RNAs, along with the full length genomic RNA. Presence or absence of the subgenomic RNAs correlated strictly with the viral strains used. The role of the defective interfering particles in measles virus vaccine attenuation, and in its seroconversion efficacy upon vaccination, as well as the potential hazard of the presence of defective interfering particles in live-virus vaccine preparations, is discussed.