SUMMARYA maturation process in the mast cells of the rat has been described. It involves an increase in cell size and granule content, accompanied by a progressive sulphation of the granules.A specific stain previously described was found to stain only mature mast cell granules, i.e., those containing the more highly sulphated forms of heparin. The immature mast cell granules, i.e. those containing heparin monosulphate, were not stained by the specific stain but were stained by aqueous methylene blue.