This chapter elaborates characterization of immune adjuvants that are defined as substances or procedures, which aid in the development or manifestations of the immune system. The protection conferred by this system against material foreign to the host ranges from microorganisms to tumors. The very complexity of the immune system introduces the possibility of multiple ways and targets by which its activities might be assisted. Several substances were described that stimulated selectively either T or B cells to transform into large blast cells and divide. The lipid moiety of the lipopolysaccharides termed Lipid A, consisting of phosphorylated glucosamine dissacharide units with ester and amide linked fatty acids, when properly dispersed was shown to be one of the most powerful mitogens for antibody synthesizing B lymphocytes in vitro. The use of nucleic acids as adjuvants to the immune response was indicated in the demonstration of a restorative capacity of nucleic acid-rich tissues in animals rendered immunodeficient following X-irradiation.