Ruthenium Red staining of ultrathin sections of human mast‐cell granules*

SUMMARY Ruthenium Red was used for staining glutaraldehyde‐fixed ultrathin sections of mast cells embedded in Epon 812. The dye was dissolved in 0.1 M ammonia, or 0.1 N acetic acid or phosphate buffer at pH 7.4, 0.1 M at a concentration of 0.5%. The ammoniacal solution stained acid mucopolysaccharides of human mast‐cell granules more specifically than did the acetic solution. The probable site of the acid mucopolysaccharides is the finely granular material within the granules.