Susceptibility of the individual is a necessary factor when considering the application of an immunizing agent. In many instances age exerts a controlling influence on susceptibility. It is well known that smallpox, erysipelas, whooping cough and chickenpox may occur in very early infancy. Any of these diseases may be encountered during the first few weeks of life; smallpox and erysipelas may develop soon after birth. Various forms of meningitis also occur very early. Diphtheria and scarlet fever are not common before 1 year of age and are rare under 6 months; the same holds true for poliomyelitis. Measles is very infrequent prior to the first half year of life. PROCEDURES FOR ACTIVE IMMUNITY AGAINST SOME OF THE MORE COMMON INFECTIOUS DISEASES In view of the foregoing facts, vaccination against smallpox should be the first protective measure to be adopted in the life of the child to afford artificial immunity. If