An improved method for staining cells of the endocrine polypeptide (APUD) series by masked metachromasia: application of the principle of 'fixation by excluded volume'.

Masked metachromasia is demonstrated by staining with a metachromatic basic dye, after acid hydrolysis of suitably fixed tissue. We report that the addition of 20% Carbowax 20M (an inert polyer, mol. wt. about 20,000) to the hydrolysis mixture improved the reaction. The improved method gives increased metachromasia , greater tolerance to variations in hydrolysis conditions, and demonstrates a greater proportion of cells--presumably due to a lower threshold of sensitivity. Lower molecular weight polymers (Carbowax 1000, Carbowax 6000) are less effective.