In a histological study of the staining effect of the staining solution Kariesdetektor 60 human teeth with carious lesions of varying sizes were separated into one control half and one test half each immediately following their extraction. Non-decalcified thin ground sections were prepared with the aid of the saw-cut-and-grind technique. The test halves were stained with Kariesdetektor, the control preparations with toluidine blue. The staining effect of the disclosing dye was evaluated under the light microscope and compared with the control preparations. In enamel the disclosing dye stained all carious areas except the dark and the more peripheral transparent zones. In dentinal caries the zones of necrosis and penetration were safely stained. Zones of demineralization, dead tracts, and transparency zones were not stained. Kariesdetektor is a reliable optical aid in removing infected carious hard tooth structures.