AbstractAn investigation, primarily of the stock Congo red solution for the staining of amyloid, was made. It was found that some samples of Congo red contained a second component which was only slightly soluble in NaCl ethanol. This made definition of- a saturated solution impossible. However, 50 mg Congo red in 50 ml of NaCl ethanol mechanically stirred for 15 minutes, then adding 0.5 ml 1% NaOH and filtering and staining in this solution for 20 minutes, gave an adequate stain for amyloid.