Saponins are widely distributed plant glycosides comprising either steroidal or triterpene aglycones linked to carbohydrate chains. The triterpene saponins from Quillaja saponaria Molina, Gypsophila sp., and Saponaria officinalis, have unique immunostimulating and immunomodulating properties. Analysis of their structure/function relationships shows common structural characteristics (i.e. a triterpene aglycone containing an aldehyde group at C-4, and oligosaccharide chains attached to positions 3 and 28) that appear to account for these saponins' effects on the immune system. The crucial role of the saponin's triterpene imine-forming carbonyl group in immune stimulating and modulating activities is supported by the following: i) saponins lacking such an aldehyde groupare devoid of these activities, and ii) modification of the quillaja saponin's aldehyde group results in the loss of such activities. The oligosaccharide chains of these saponins appear to mediate targeting of saponins to antigen presenting cells(APCs), thus enhancing their immunological properties. The saponins from Quillaja saponaria Molina also contain two acyl moieties: a normonoterpene carboxylic acid and a normonoterpene carboxylic acid glycoside, which are linked linearly to a fucosyl residue attached at position C-28. These acyl groups appear to be responsible for the stimulation of cytotoxic T cells (CTLs) against exogenous antigens. Removal of the acyl moieties by mild basic hydrolysis yields deacylated quillaja saponins that are structurally and functionally comparable to the non- acylated Gypsophila sp. and Saponaria officinalis saponins; all these non-acylated saponins stimulate a poor primary immune response. Replacement of the quillaja saponin's acyl moieties with other hydrophobic groups alters their immune stimulating and modulating properties. © 2000 Elsevier B.V. All rights reserved.