Follicles displaying a positive periodic acid-Schiff reaction to a colloid substance in the anterior pituitary gland have been observed in many vertebrates, including humans. It is also known that these follicles greatly increase in number and size with age. We therefore performed histochemical analysis of these colloid follicles in the senescent porcine pituitary gland in order to clarify their nature and biological significance. Results from various histological stains suggest that the colloid contains some polysaccharides or glycoproteins. In addition to the histological stains, lectin histochemistry revealed that the colloid contains sialic acid, N-acetyl galactosamine and galactose. Also, lectin staining indicated that some glycoproteins, most likely Asn-linked sialoglycoproteins, are localized in the colloid. The cells (FS cells) surrounding the colloid were densely stained with an antibody to S-100 protein but were not stained with antibodies to any other anterior pituitary hormones. Frequently mammotrophs (PRL cells), and occasionally gonadotrophs (LH cells) were found closely associated with folliculo-stellate cells (FS cells) which lay next to the large colloid containing follicles. This suggests that not only are the FS cells important in the production of the colloids, but the adjacent LH and PRL cells in some way also contribute to their formation.