The modern concept of the mode of action of adjuvants has evolved in exact relation to our understanding of the lymphoid organ system and the complexity of the immune response and its regulation. In the late 60's, attention remained focussed primarily on 'the antibody response' and on the physical attributes of adjuvants which enhanced this response. A major symposium published in 1967 [145] emphasized: the value of depot formation, with slow release of antigen and development of a local granuloma; the value of particles bearing antigenic determinants; the relative merits of water-in-oil emulsions, and the usefulness of metabolizable oils; the apparent importance of lipophilic or surface active materials as adjuvants; and the striking activity of cationic quarternary ammonium compounds with long alkyl side chains.Unquestionably the first insight into the selective or differential action of adjuvants on immune responses was Dienes' demonstration in the late 20's that simple protein antigens preferentially induce CMI (cell-mediated immunity, delayed hypersensitivity) when injected into a tuberculous focus [47]. This theme was continued in the early studies of Freund's adjuvant [60] and more recent studies of tubercle bacillus derivatives such as MDP (muramyl dipeptide) [25]. White, in the 1967 symposium, demonstrated that mycobacterial adjuvants which intensified the cell-mediated response also increased 72 antibody formation relative to 71 (in the guinea pig). We now know both of these to be intensely thymusdependent responses, and current research on MDP, for example, is focussed on the action of this compound directly on various subpopulations of T-lymphocytes in culture.