The hemagglutination inhibition (HAI) test is not mathematically precise and reproducible. Thus, it is critical to know whether or not the threshold titer (1 : 10) of detectable rubella antibody is indicative of true immunity. Three patients with a 1 : 10 HAI titer presented with subsequent rubella during gestation. Ninety postpartum patients with a HAI titer of 1 : 10 were vaccinated and the rubella antibody titers were reassessed. Seventeen percent of these patients responded to the vaccine challenge with an eightfold or greater rise in titer. Thus, we recommend that the patient with a low HAI titer (1 : 10) should be considered to have marginal immunity to rubella and should be vaccinated.