Antibody avidity of human anti-rubella antibodies has been determined using the chaotropic thiocyanate ion in ELISA assays. Independence of antibody level and affinity was shown and the pattern of post-vaccination antibody affinity established. A variation of the standard ELISA assay was developed where replicate wells containing antibody bound to antigens were exposed to increasing concentrations of the chaotropic thiocyanate ion. Resistance to thiocyanate elution was utilised as the measure of avidity and an index (affinity index) representing 50% of effective antibody binding was used to compare different sera. The interplate coefficient of variation for the affinity index was 12.6% whereas the intraplate coefficient of variation was 4.1%.