Immunomodulating activity of triterpene glycosides of the holostan series (holoturins A and B, holoturin A2 and holotoxin A1) and triterpene glycosides of the dammaran series (3-O-monoglycoside,12-O-monoglycoside and 20-O-monoglycoside of protopanaxadiol and 3-O-monoglycoside of betulafolientriol) was studied in vitro. In low concentrations the triterpene glycosides showed mitogenic activity and modulated the immune response. The similarity in the action of the glycosides was first of all observed with respect to the dose-dependent duality of their effects i.e. the diametrically opposite action of the high and low doses. The expression of the effects was likely determined by the chemical structure of the triterpene glycosides. Liberation of the soluble mediators served as a secondary signal to the clonal expansion and differentiation of the cells.