By intensive stimulation with large amounts of Salmonella flagellar antigen, newborn rabbits were induced to form high titer flagellar agglutinins usually by the 7th to 10th day of life. Characterization of the agglutinins at various times during the first 30 days of life revealed that the earliest antibody which appeared was a gamma-1 macroglobulin, and that 7S gamma-2 globulins did not appear until the 4th or 5th week of life. In contrast, the adult animals produced macroglobulin antibodies for only 3 to 5 days before the lower molecular weight variety appeared. The infant macroglobulin appears to be similar in all respects to adult macroglobulin antibodies.

These data are interpreted to indicate that the newborn and adult rabbit differ in their response to this type of stimulus not in timing of macroglobulin antibody production, but chiefly in the prolonged interval, which precedes the development of the capacity for the 7S type response in the newborn animal.