During cavity excavation diagnostic test are used to prevent any unnecessary removal of sound dentine. In this study the validity of a newly introduced blue dye (Quadrant CariTest) was investigated. Twenty-five carious teeth were collected immediately after extraction. The caries lesions were stepwise excavated. From four excavation steps per lesion a digital and an ordinal blue measurement was obtained. From each excavation phase a little amount of excavated dentine was cultured on blood plates. The correlation coefficients between the ordinal and the digital blue measurements and the counts of micro-organisms, expressed in log(CFU)/ml, were respectively 0.56 and 0.45. The sensitivity and the specificity of the ordinal blue measurements were 0.69 and 0.83, those of the digital blue measurements 0.76 and 0.65. The area under ROC of the ordinal blue measurements was 0.77, and that of digital blue measurements 0.70. The areas under ROC were not significantly different (p = 0.14). It was concluded that the new caries indicator is well applicable during the cavity preparation to demonstrate infected dentine.