Forty infants from 1 week to 6 months of age who had no circulating antitoxin were immunized with one injection of a high titer diphtheria toxoid on alum and their antitoxin responses were followed at regular intervals up to six months afterwards. The ability to form antitoxin improved significantly during the first two months of life. No significant difference could be demonstrated between the response of infants 2 to 4 months of age and that of infants 6 months of age. (In the statistical tests, the 5% level was adopted as indicating significance.) Twenty-seven of these infants were immunized at the same time in the opposite arm with a single dose of tetanus toxoid on alum. The response to tetanus immunization was faster and better in infants from 2 to 6 months of age than in infants 1 month of age and younger. The response to the tetanus toxoid preparation was faster and better than the response to the diphtheria toxoid preparation. The following conclusions are drawn: A. The infant can form antibodies from birth. B. The ability to form antibodies improves during early life. For diphtheria and tetanus antitoxin, the improvement is rapid during the first two months and then is much slower. C. The advisability of early immunization is related to the potency of the antigen to be used.