In this study we demonstrate that iscoms made with different defined fractions of quillaja saponins (fractions A and C) exhibit strikingly different immunomodulatory activities. Fraction A has a potent immunomodulatory activity by enhancing antigen specific proliferation and production of IL-2 and IFN-gamma, interestingly, this is not reflected by increased levels of IgG2a. The C-fraction on the other hand strongly enhance antibody production and has the ability to modulate the IgG subclass pattern towards IgG2a, in spite of the fact that C-iscoms induce only low levels of IFN-gamma. Iscoms containing both fractions A and C (Iscoprep703) induced higher serum antibody responses than iscoms made of either of the fractions alone and showed an enhanced ability to modulate the serum subclass pattern towards IgG2a compared to C-iscoms.