Immunization of newborn infants with Salmonella vaccines results in agglutinin formation to the flagellar antigen as early as 7 days of life, and in 80% of infants by 14 days. This agglutinin is a gamma-l macroglobulin with a sedimentation rate of approximately 17 to 20S, and is susceptible to reductive cleavage. Production of a 7S sedimenting gamma-2 globulin agglutinin occurred by the 30th or 40th day after immunization in some infants. In the adult, macroglobulin agglutinins against the flagellar antigen appeared 4 to 5 days after immunization, followed by appearance of 7S agglutinins 3 or 4 days later. Both the macroglobulin and the slower sedimenting gamma-2 globulin agglutinin were thus qualitatively similar in the adult and infant and differed only in the timing of their appearance. Transplacentally acquired agglutinins specifically inhibited the response to the homologous flagellar antigen. Evidence was found for an agglutinin of intermediate sedimentation rate which had the electrophoretic mobility of gamma-l globulin, and was susceptible to reductive cleavage. This type agglutinin appeared after the macroglobin and before the 7S agglutinin. These studies support the concept that immunity, like all developmental processes, must not be measured to adult norms. At each stage of ontogeny, the mechanisms which have evolved should be judged by their appropriateness in dealing with the threats to survival peculiar to the epoch rather than those of full maturity.