BACKGROUND The compositional nature of the pigment of melanosis coli is essentially unknown. Previous histochemical studies suggested that this pigment has certain similarities with lipofuscin (i.e., age-dependent pigment) and ceroids (i.e., pathologically derived pigments) and that it may contain, therefore, polymerized glycolipids and glycoproteins. However, the saccharide composition of this pigment was never explored by lectin histochemical procedures, which was the main object of this study. METHODS Colonoscopic biopsy specimens from eight patients with melanosis coli and from three normal control subjects were studied by fluorescent microscopy and by standard and lectin histochemistry. The number of apoptoses in the lining colonic epithelium was also evaluated histologically. RESULTS Apoptotic bodies were significantly more numerous in patients with melanosis coil than in control subjects. The pigment that accumulates in macrophages of the lamina propia showed autofluorescence, sudanophilia, acid-fastness, and positiveness to PAS and Schmorl's reactions, all of which are common to lipofuscin and ceroids, plus an intense argentaffin reaction abolished by bleaching, indicative of a melanic substance. Lectin histochemistry showed, in decreasing order of frequency, the presence of alpha-D-mannose, sialic acid, beta-D-galactose (lactose), gal-beta-(1-3)acetyl-galactosamine, alpha-D-galactose, and alpha-L-fucose, but no terminal alpha-D-acetyl-galactosaminyl residues. CONCLUSIONS The significant increase of apoptotic bodies in the lining colonic epithelium indicated that this type of cell death is not due to the natural programmed cell renewal, but to the action of laxatives. Because the autofluorescent pigment of melanosis coli contains melanin (as well as glycoconjugates) and is not dependent on age but on the use of anthranoid laxatives, it should be categorized as a "melanized ceroid." The lectin affinities of this pigment indicated that it contains a substantial number of saccharide residues almost similar to those found in the ceroid pigment of human aortic atheromas. These findings and considerations on the metabolism and pharmacokinetics of anthranoids suggested that the apoptotic epithelial cells, rather than the laxatives, may be the source of the pigment saccharides, whereas the precursors of the melanic substance may be derived from the anthranoids.