The level of natural hemagglutinins for several mammalian erythrocytes was low in embryos 12 to 20 days of age and in baby chicks up to two weeks of age. The antibody level increased rapidly during the third and fifth week after hatching and at a slower rate thereafter with age. The agglutinin levels were highest for rabbit and rat erythrocytes, intermediate for mouse and hamster cells, and lowest for sheep red cells.The level of immune antibody in the blood during the primary responses to sheep, mouse and hamster erythrocytes (1) reached a peak about 6 days after antigen injection, ( 2 ) was higher when antigen was injected intravenously as compared with the intraperitoneal route, (3) varied with antigen dose and, (4) increased with the age of the bird. Moreover, the antibody forming capacity was drastically reduced by x-irradiation or cyclophosphamide treatment of embryos or baby chicks.Employing approximately optimum antigen doses, it was found that 14-to 18-day embryos were unresponsive, whereas some newly hatched chicks and practically all week-old chicks were capable of producing hemagglutinins to five mammalian erythrocytes. Associated with this change in competence, there is a n increase in number of germinal centers in the spleen. The antibody forming potential increased with age, growing most rapidly during the first month and slowly thereafter. The possible relationship between the growth of the primary antibody forming potential and the occurrence of natural antibodies has been discussed.