An important consideration in evaluating vaccines is the duration of immunity. The only really important measure of this immunity is the protection against infections and/or illness at various time intervals, following natural or artificial challenge. There are few data of this sort, more commonly immunity is estimated by measuring serum antibody, in many instances an erroneous measure. Serum antibody levels to respiratory viruses fall only slightly 6 months following infection or immunization. It is difficult to assess the duration of antibody for much longer than this, because of problems with intercurrent infection. With respiratory bacterial infections, e.g. pneumococcal pneumonia, parenterally-induced immunity probably lasts for only several months. Secretory antibody induced by inactivated viral vaccines, seems to persist for about a year, after having reached a peak level at about 4-6 weeks following immunization. Work with the live attenuated polio virus vaccine indicates longer lasting immunity, with detectable antibody persisting for up to 34 months. Restimulation with the inactivated polio virus vaccine produced no evidence of a secondary response (memory). Following booster immunization with influenza very little evidence of memory is seen. Cell-mediated immunity (CMI): in guinea pigs BCG sensitization can be demonstrated for at least 2-9 months. In humans, intracutaneous BCG immunization leads to positive tuberculin reaction in 6-10 weeks, and skin sensitivity lasts an average of about 4 years. There is contradicting data as to the duration of protection against infection following BCG immunization. Local and systemic CMI have been shown to exist independently of each other in experimental animals and man.