Immune stimulating complexes (iscoms) are 30-40 nm cage-like structures, which consist of glycoside molecules of the adjuvant Quil A, cholesterol and phospholipids in which antigen can be integrated. Even in the presence of pre-existing antibodies they may function as a potent adjuvant system by inducing high systemic and local long-lasting antibody and T cell responses. An additional advantage is their capacity to induce MHC class I restricted CD8+ CTL responses. This combination will in most cases, when the relevant antigens are incorporated, lead to adequate protection against virus infection or disease. An overview is given of the present data available on the use of iscoms in viral systems, and some relevant examples are discussed to highlight general principles in the use of iscoms as an adjuvant system.