The mechanism of action of anthranoids in general and of sennosides at the cellular level is not precisely known. Pseudomelanosis or pseudolipofuscinosis, a condition characterized by the accumulation of pigmented macrophages in the lamina propria, is one of the well-known effects of these products. It is most probably the result of an interaction between apoptotic epithelial cells and the lamina propria cellular infiltrate. Treatment of cell suspensions of intestinal epithelial cells and of human intestinal epithelial cells in culture with rhein anthrone, the active compound of sennosides, demonstrates a direct influence of the drug on these epithelial cells. Low doses induce alterations in cellular shape and organelles consistent with increased metabolism. High doses induce apoptotic changes. The interaction between the epithelial cells and cells of the monocyte/macrophage lineages induces also the release of prostaglandins of the E series as shown by experiments on cell cultures of epithelial cells and peripheral blood cells. An increase of PGE2 release to about 140% of the control value is noted following administration of low doses of rhein anthrone to a combination of human intestinal epithelial cells and human peripheral blood mononuclear cells. This finding indicates that rhein anthrone is activating cellular components of the intestinal immune system and may by this pathway induce secretion and motility.