Histochemical and ultrastructural observations were carried out on pigments in melanosis coli from a 71 years old male who has taken laxatives(laxenta® and dulcolax®) for 3 years and received a left hemicolectomy on adenocarcinoma of the descending colon. The pigments were confined within the lamina propria of entire non-neoplastic colonic mucosa and showed histochemical reactions similar predominantly to lipofuscin but with partial overlapping with those in melanin pigments. Ultrastructurally, the denatured mitochondria and endoplasmic reticulum were demonstrated within the pigmented granules and in the macrophages to suggest that pigments are of lipofuscin derivatives which originate from the denatured lipoprotein membranes of cytoplasmic organelles of injured colonic epithelial cells.