The purpose of study was to investigate the variation of validity of the Resazurin Disc Caries Activity Test by subdivision of the measuring scale of the test. The 3-level color scale of RD test was revised and 4- and 5-level scales were made. The subjects of study were preschool children, 227 for 3-level group, 238 for 4-level group, and 256 for 5-level group. The validity of the RD test using each scale was analyzed by using the caries experience as criteria. The results were as follows: