Nonreplicating, purified subunit or synthetic viral vaccines of the future are likely to be weak immunogens that will require immunopotentiation if they are to be effective. These marginal vaccinescould be improved by combination with potent and safe immunologic adjuvants. The use of adjuvants should also reduce the amount of purified antigen required for successful immunization, thus making vaccine production'rnore economical and more feasible. It may be possible to combine the recently developed, relatively nontoxic synthetic immunoregulators of low molecular weight with antigens in order to modulate preselected compartments of the immune system. To date, the question of adjuvant safety has not been resolved and represents the major obstacle to the orderly development of adjuvanted vaccines. The fear of inducing cancer and other immediate or long-term perturbations of the immune system must be patiently and rationally overcome by basic and applied experimentation and by the development of appropriate guidelines for studies in humans.