An - You didn't mention what cells you were staining, but I assume it's mast cells. Thionin stains are very pH dependent. For mast cells, the pH should be 1. At that pH only mast cells stain. Cheryl Cheryl Crowder, BA, HTL(ASCP) Crowder Histology Consulting 4952 Alvin Dark Ave. Baton Rouge, LA 70820 (225) 772-2865