QS-21 and QS-7 are two adjuvant-active saponins that can be obtained in high purity from Quillaja saponaria Molina extracts. QS-21 is a highly characterized compound and is known to be a potent adjuvant for antibody and CD8+ CTL response to subunit antigens. Less is known about the activity and structure of the hydrophilic saponin QS-7. Hence, we have carried out a detailed structural and immunological characterization. As with QS-21, QS-7 was shown to be a 3,28-O-bisglycoside quillaic acid, with some differences being a higher degree of glycosylation and a considerably shorter fatty acyl unit in QS-7. These differences were correlated to a lower lytic activity against sheep red blood cells. Different doses of QS-7 were evaluated for stimulation of immune response to the antigen ovalbumin, given three times by subcutaneous route to C57BL/6 mice. QS-7 doses of 40 micrograms or higher were shown to induce a strong CD8+ CTL response reproducibly against E. G7-OVA targets (similar to that induced by a 5-10 micrograms dose of QS-21). QS-7 (at doses above 5 micrograms) was also shown to stimulate CTL against peptide 18 of HIV-1IIIB gp120 after three immunizations of Balb/c mice with recombinant gp120 and different doses of QS-7. These data suggest that a hydrophilic saponin with low lytic activity can stimulate MHC Class I CTL responses although a higher minimum dose may be required for some antigens.