The capacity of the human immune system to mount an antibody response following in vivo immunization with a protein or polysaccharide antigen is a revealing indication of the overall integrity of both the B and T cell arms of the immune system. As such, in vivo immunization followed by measurement of the antibody response is an appropriate test of immune function in the various acquired and congenital immunodeficiencies and in a host of other conditions affecting the immune system. This unit describes procedures for in vivo immunization and for the measurement of the subsequent immune response using an ELISA technique.