Two new cardenolide glycosides (1 and 2), along with six known cardenolide glycosides (3-8), have been isolated from the roots of Pergularia tomentosa. In order to investigate their potential anticancer activity, these compounds were tested in an in vitro growth inhibitory assay (a MTT colorimetric assay), including six different human cancer cell lines, and for their ability to inhibit Na(+)/K(+)-ATPase activity, in addition to the morphologic changes induced in human cancer cell lines (using computer-assisted phase-contrast microscopy). The data revealed that these cardenolides displayed marked cytotoxic activity. The results obtained suggest that structural characteristics of the cardenolides studied, with the A/B rings of the steroidal skeleton trans fused and containing a single sugar in a unique "dioxanoid" attachment, confer on them specific cytotoxic properties that are distinct from those displayed by classic cardenolides such as digoxin.