A threshold (1:10) rubella antibody [Ab] hemagglutination inhibition (HAI) titer was obtained for 288 of 6537 (4.4%) obstetric patients. Random sera from 84 of these patients were compared for rubella Ab by HAI and a very sensitive radioimmunoassay (RIA) technique. By RIA, 17% of the sera had no detectable rubella Ab. Those patients apparently were truly susceptible to rubella. Of 55 paired pre-vaccine and post-vaccine sera with titer increases demonstrated by HAI, 8 were analyzed by RIA. Of the 8 pre-vaccine sera, 7 had no rubella virus Ab by RIA and RIA showed seroconversion in all 8 post-vaccine sera.