Sensitivity to rubella virus hemagglutination differs considerably with newborn as opposed to adult erythrocytes (RBC) in several animal species. In humans, cord RBC are 16 times more sensitive to hemagglutination, whereas hemagglutination titers with newborn RBC are 6 to 16 times higher in chickens, mice, and rabbits. This higher sensitivity is, however, short-lived, and adult RBC values are observed 7 days after birth in mice. Trypsinization has no effect on newborn RBC, but increases sensitivity of adult RBC to that of neonatal RBC and tends to diminish differences among individuals. On the other hand, trypsinization does not further enhance the very high sensitivity of pigeon, duck, and goose RBC or the low sensitivity of both newborn and adult guinea pig RBC. Human cord RBC proved suitable for use in hemagglutination-inhibition tests, since they gave titers comparable to those obtained with other types of RBC and offered some advantages: by comparison with animal RBC, they did not require adsorption to remove nonspecific agglutinins; by comparison with adult human RBC, they did not need trypsinization.