IN VITRO and in vivo properties of the attenuated Cendehill strain of rubella virus have been described previously. This strain has been attenuated by serial propagation in primary rabbit kidney cells. It is characterized by the production of an early cytopathic effect in rabbit cultures, by the formation of distinct plaques in RK13cells, by loss of intranasal infectivity for the monkey, and by loss of immunogenicity for the rabbit after subcutaneous inoculation.1,2Preliminary clinical trials with the Cendehill strain at the 21st and 51st passage levels have been reported previously.3-6 Although attenuated virus at the 21st passage failed to spread from vaccinated subjects to susceptible contacts, the trials with this passage level were discontinued because mild but typical symptoms of rubella were observed in one of the vaccinees.4We report here the results of several clinical trials in children and young adults, with the high-passage