Lipopeptides constitute potent novel immunoadjuvants in mice, rabbits and other species, enhancing markedly the immune response when given in mixture with antigens. Lipopeptides are non-toxic, non-pyrogenic and do not induce tissue damage when injected. They are thus well suited to replace Freund's adjuvant avoiding its side effects; the antibody titres obtained using lipopeptide analogues are in most cases comparable to the titres obtained by administering Freund's adjuvant. Lipopeptides also improve the efficiency of vaccines, which is important in decreasing the amount of vaccine required. Lipopeptides covalently coupled to low molecular weight haptens, e.g. peptides or toxins, are able to elicit high antigen-specific antibody titres in mice and rabbits. Conjugates containing B or T helper cell epitopes constitute novel synthetic vaccines which protect against viral infections by inducing virus-specific antibodies. When coupled to CTL epitopes, the conjugates are able to induce cytotoxic T lymphocytes in vivo which eliminate virus-infected cells. Thus, due to their efficacy and their lack of side effects, these novel lipopeptide adjuvants provide a substitute for many conventional adjuvants.