The immunogenicity and clinical reactivity of rubella vaccine derived from WI-38 or MRC5 human diploid cells was compared in 125 seronegative adolescent females. Seroconversion rates, assessed by single radial haemolysis testing of paired pre- and post-vaccination samples exceeded 98% (56/57 and 68/68 vaccinees, respectively) for both vaccines. Quantitative assessment of rubella-specific antibodies in 53 post-vaccination sera by an ELISA technique also failed to reveal any difference in immunogenicity between the vaccines. Assessable calendar records documenting the occurrence of local and systemic signs and symptoms in the four weeks following vaccination were returned by 106 subjects. No important statistically significant difference in parameters of clinical reactivity between the vaccine groups was observed although the incidence of pain at the injection site was found to be significantly higher for vaccinees receiving WI-38 derived vaccine.