Epitopes on microbial antigens responsible for protective immunity have begun to be identified and isolated, and their chemical structures have been determined. Ensuing knowledge of their weak immunizing capacity per se has led to an appreciation of the need for adjuvants to increase the immunogenicity of these low-molecular weight synthetic structures. As such, a recent surge in adjuvant research has emerged. Accordingly, this review will highlight a number of those adjuvant substances whose activity in animals indicates a potential use in human vaccines. In addition, the potential of several well-defined substances, termed immunomodulators, which nonspecifically stimulate resistance of animals to multiple 50% lethal doses of microbial challenge is described. Among the most extensively characterized adjuvants of microbial origin discussed in detail are (i) the lipopolysaccharides isolated from gram-negative bacteria and their nontoxic analogs, (ii) the synthetic muramyl dipeptides and their multiple analogs, and (iii) the synthetic polyribonucleotide complexes, mimicking the interferon-inducing capacity of viruses. Discussed also are the heat-labile enterotoxin of Escherichia coli, the nonionic block copolymers, the saponins, a quinolamine derivative, and the hormone dihydroepiandrosterone.